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[PATCH v9 2/2] handle relation statistics correctly during rewrites
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* [PATCH v9 2/2] handle relation statistics correctly during rewrites
@ 2025-11-04 13:52  Bertrand Drouvot <[email protected]>
  0 siblings, 0 replies; 333+ messages in thread

From: Bertrand Drouvot @ 2025-11-04 13:52 UTC (permalink / raw)

Now that PGSTAT_KIND_RELATION is keyed by refilenode, we need to handle rewrites.

To do so, this patch:

- Adds PgStat_PendingRewrite, a new struct to track rewrite operations within
a transaction, storing the old locator, new locator, and original locator (for
rewrite chains). This allows stats to be copied from the original location to
the final location at commit time.

- Adds a new function, pgstat_mark_rewrite(), called when a table rewrite begins.
It records the rewrite operation in a local list and detects rewrite chains by
checking if the old_locator matches any existing new_locator, preserving the
chain's original_locator.

- Modifies pgstat_copy_relation_stats(), to accept RelFileLocators instead of
Relations, with a new increment parameter to accumulate stats (needed for rewrite
chains with DML between rewrites).

- Ensures that AtEOXact_PgStat_Relations(), AtPrepare_PgStat_Relations(),
pgstat_twophase_postcommit()/postabort() pgstat_drop_relation() handle the
PgStat_PendingRewrite list correctly.

Note that due to the new flush call in pgstat_twophase_postcommit() we can not
call GetCurrentTransactionStopTimestamp() in pgstat_relation_flush_cb(). So,
adding a check to handle this special case and call GetCurrentTimestamp() instead.
Note that we'd call GetCurrentTimestamp() only if there is a rewrite, so that
the GetCurrentTimestamp() extra cost should be negligible. Another solution
could be to trigger the flush from FinishPreparedTransaction() but that's not
worth the extra complexity.

The new pending_rewrites list is traversed in multiple places. The overhead
should be negligible in comparison to a rewrite and the list should not contain
a lot of rewrites in practice.

The pending_rewrites list is traversed in multiple places. In typical usage,
the list will contain only a few entries so the traversal cost is negligible (
furthermore in comparison to a rewrite).
---
 src/backend/catalog/index.c                  |   2 +-
 src/backend/commands/cluster.c               |   5 +
 src/backend/commands/tablecmds.c             |   6 +
 src/backend/utils/activity/pgstat_relation.c | 391 ++++++++++++++++++-
 src/backend/utils/activity/pgstat_xact.c     |  25 +-
 src/backend/utils/cache/relcache.c           |   6 +
 src/include/pgstat.h                         |   5 +-
 src/tools/pgindent/typedefs.list             |   1 +
 8 files changed, 424 insertions(+), 17 deletions(-)
  92.8% src/backend/utils/activity/
   4.9% src/backend/

diff --git a/src/backend/catalog/index.c b/src/backend/catalog/index.c
index 8dea58ad96b..b71925a22c3 100644
--- a/src/backend/catalog/index.c
+++ b/src/backend/catalog/index.c
@@ -1795,7 +1795,7 @@ index_concurrently_swap(Oid newIndexId, Oid oldIndexId, const char *oldName)
 	changeDependenciesOn(RelationRelationId, oldIndexId, newIndexId);
 
 	/* copy over statistics from old to new index */
-	pgstat_copy_relation_stats(newClassRel, oldClassRel);
+	pgstat_copy_relation_stats(newClassRel->rd_locator, oldClassRel->rd_locator, false);
 
 	/* Copy data of pg_statistic from the old index to the new one */
 	CopyStatistics(oldIndexId, newIndexId);
diff --git a/src/backend/commands/cluster.c b/src/backend/commands/cluster.c
index 2120c85ccb4..6155b12afab 100644
--- a/src/backend/commands/cluster.c
+++ b/src/backend/commands/cluster.c
@@ -1196,6 +1196,11 @@ swap_relation_files(Oid r1, Oid r2, bool target_is_pg_class,
 
 		rel1 = relation_open(r1, NoLock);
 		rel2 = relation_open(r2, NoLock);
+
+		/* Mark that a rewrite happened */
+		if (RELKIND_HAS_STORAGE(rel1->rd_rel->relkind))
+			pgstat_mark_rewrite(rel1->rd_locator, rel2->rd_locator);
+
 		rel2->rd_createSubid = rel1->rd_createSubid;
 		rel2->rd_newRelfilelocatorSubid = rel1->rd_newRelfilelocatorSubid;
 		rel2->rd_firstRelfilelocatorSubid = rel1->rd_firstRelfilelocatorSubid;
diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index 6b1a00ed477..9de70f321ed 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -16884,6 +16884,7 @@ ATExecSetTableSpace(Oid tableOid, Oid newTableSpace, LOCKMODE lockmode)
 	Oid			reltoastrelid;
 	RelFileNumber newrelfilenumber;
 	RelFileLocator newrlocator;
+	RelFileLocator oldrlocator;
 	List	   *reltoastidxids = NIL;
 	ListCell   *lc;
 
@@ -16922,6 +16923,7 @@ ATExecSetTableSpace(Oid tableOid, Oid newTableSpace, LOCKMODE lockmode)
 	newrlocator = rel->rd_locator;
 	newrlocator.relNumber = newrelfilenumber;
 	newrlocator.spcOid = newTableSpace;
+	oldrlocator = rel->rd_locator;
 
 	/* hand off to AM to actually create new rel storage and copy the data */
 	if (rel->rd_rel->relkind == RELKIND_INDEX)
@@ -16934,6 +16936,10 @@ ATExecSetTableSpace(Oid tableOid, Oid newTableSpace, LOCKMODE lockmode)
 		table_relation_copy_data(rel, &newrlocator);
 	}
 
+	/* mark that a rewrite happened */
+	if (RELKIND_HAS_STORAGE(rel->rd_rel->relkind))
+		pgstat_mark_rewrite(oldrlocator, newrlocator);
+
 	/*
 	 * Update the pg_class row.
 	 *
diff --git a/src/backend/utils/activity/pgstat_relation.c b/src/backend/utils/activity/pgstat_relation.c
index 48bf93cae6e..b6ba8fdf440 100644
--- a/src/backend/utils/activity/pgstat_relation.c
+++ b/src/backend/utils/activity/pgstat_relation.c
@@ -30,6 +30,19 @@
 #include "utils/syscache.h"
 #include "utils/timestamp.h"
 
+/* Pending rewrite operations for stats copying */
+typedef struct PgStat_PendingRewrite
+{
+	RelFileLocator old_locator;
+	RelFileLocator new_locator;
+	RelFileLocator original_locator;
+	int			nest_level;		/* Transaction nesting level where rewrite
+								 * occurred */
+	struct PgStat_PendingRewrite *next;
+} PgStat_PendingRewrite;
+
+/* The pending rewrites list for current transaction */
+static PgStat_PendingRewrite *pending_rewrites = NULL;
 
 /* Record that's written to 2PC state file when pgstat state is persisted */
 typedef struct TwoPhasePgStatRecord
@@ -43,6 +56,8 @@ typedef struct TwoPhasePgStatRecord
 	PgStat_Counter deleted_pre_truncdrop;
 	RelFileLocator locator;		/* table's rd_locator */
 	bool		truncdropped;	/* was the relation truncated/dropped? */
+	RelFileLocator rewrite_old_locator;
+	int			rewrite_nest_level;
 } TwoPhasePgStatRecord;
 
 
@@ -54,27 +69,70 @@ static void restore_truncdrop_counters(PgStat_TableXactStatus *trans);
 
 
 /*
- * Copy stats between relations. This is used for things like REINDEX
+ * Copy stats between RelFileLocator. This is used for things like REINDEX
  * CONCURRENTLY.
  */
 void
-pgstat_copy_relation_stats(Relation dst, Relation src)
+pgstat_copy_relation_stats(RelFileLocator dst, RelFileLocator src, bool increment)
 {
 	PgStat_StatTabEntry *srcstats;
 	PgStatShared_Relation *dstshstats;
 	PgStat_EntryRef *dst_ref;
 
-	srcstats = pgstat_fetch_stat_tabentry_ext(RelationGetRelid(src));
+	srcstats = (PgStat_StatTabEntry *) pgstat_fetch_entry(PGSTAT_KIND_RELATION,
+														  src.dbOid,
+														  RelFileLocatorToPgStatObjid(src));
 	if (!srcstats)
 		return;
 
 	dst_ref = pgstat_get_entry_ref_locked(PGSTAT_KIND_RELATION,
-										  dst->rd_rel->relisshared ? InvalidOid : MyDatabaseId,
-										  RelationGetRelid(dst),
+										  dst.dbOid,
+										  RelFileLocatorToPgStatObjid(dst),
 										  false);
 
 	dstshstats = (PgStatShared_Relation *) dst_ref->shared_stats;
-	dstshstats->stats = *srcstats;
+
+	if (!increment)
+		dstshstats->stats = *srcstats;
+	else
+	{
+		/* Increment those statistics */
+#define RELFSTAT_ACC(fld, stats_to_add) \
+	(dstshstats->stats.fld += stats_to_add->fld)
+		RELFSTAT_ACC(numscans, srcstats);
+		RELFSTAT_ACC(tuples_returned, srcstats);
+		RELFSTAT_ACC(tuples_fetched, srcstats);
+		RELFSTAT_ACC(tuples_inserted, srcstats);
+		RELFSTAT_ACC(tuples_updated, srcstats);
+		RELFSTAT_ACC(tuples_deleted, srcstats);
+		RELFSTAT_ACC(tuples_hot_updated, srcstats);
+		RELFSTAT_ACC(tuples_newpage_updated, srcstats);
+		RELFSTAT_ACC(live_tuples, srcstats);
+		RELFSTAT_ACC(dead_tuples, srcstats);
+		RELFSTAT_ACC(mod_since_analyze, srcstats);
+		RELFSTAT_ACC(ins_since_vacuum, srcstats);
+		RELFSTAT_ACC(blocks_fetched, srcstats);
+		RELFSTAT_ACC(blocks_hit, srcstats);
+		RELFSTAT_ACC(vacuum_count, srcstats);
+		RELFSTAT_ACC(autovacuum_count, srcstats);
+		RELFSTAT_ACC(analyze_count, srcstats);
+		RELFSTAT_ACC(autoanalyze_count, srcstats);
+		RELFSTAT_ACC(total_vacuum_time, srcstats);
+		RELFSTAT_ACC(total_autovacuum_time, srcstats);
+		RELFSTAT_ACC(total_analyze_time, srcstats);
+		RELFSTAT_ACC(total_autoanalyze_time, srcstats);
+#undef RELFSTAT_ACC
+
+		/* Replace those statistics */
+#define RELFSTAT_REP(fld, stats_to_rep) \
+	(dstshstats->stats.fld = stats_to_rep->fld)
+		RELFSTAT_REP(lastscan, srcstats);
+		RELFSTAT_REP(last_vacuum_time, srcstats);
+		RELFSTAT_REP(last_autovacuum_time, srcstats);
+		RELFSTAT_REP(last_analyze_time, srcstats);
+		RELFSTAT_REP(last_autoanalyze_time, srcstats);
+#undef RELFSTAT_REP
+	}
 
 	pgstat_unlock_entry(dst_ref);
 }
@@ -136,6 +194,7 @@ void
 pgstat_assoc_relation(Relation rel)
 {
 	RelFileLocator locator;
+	PgStat_TableStatus *pgstat_info;
 
 	Assert(rel->pgstat_enabled);
 	Assert(rel->pgstat_info == NULL);
@@ -164,14 +223,54 @@ pgstat_assoc_relation(Relation rel)
 		locator.relNumber = rel->rd_id;
 	}
 
+	/*
+	 * If this relation was rewritten during the current transaction we may be
+	 * reopening it with its new RelFileLocator. In that case, continue using
+	 * the stats entry associated with the old locator rather than creating a
+	 * new one. This ensures all stats from before and after the rewrite are
+	 * tracked in a single entry which will be properly copied to the new
+	 * locator at transaction commit.
+	 */
+	if (pending_rewrites != NULL)
+	{
+		PgStat_PendingRewrite *rewrite;
+
+		for (rewrite = pending_rewrites; rewrite != NULL; rewrite = rewrite->next)
+		{
+			if (locator.dbOid == rewrite->new_locator.dbOid &&
+				locator.spcOid == rewrite->new_locator.spcOid &&
+				locator.relNumber == rewrite->new_locator.relNumber)
+			{
+				pgstat_info = pgstat_prep_relation_pending(rewrite->old_locator);
+				goto found_entry;
+			}
+		}
+	}
+
 	/* Else find or make the PgStat_TableStatus entry, and update link */
-	rel->pgstat_info = pgstat_prep_relation_pending(locator);
+	pgstat_info = pgstat_prep_relation_pending(locator);
+
+found_entry:
+	rel->pgstat_info = pgstat_info;
+
+	/*
+	 * For relations stats, we key by physical file location, not by relation
+	 * OID. This means during operations like ALTER TYPE it's possible that
+	 * the relation OID changes but the relfilenode stays the same (no actual
+	 * rewrite needed). Unlink the old relation first.
+	 */
+	if (pgstat_info->relation != NULL &&
+		pgstat_info->relation != rel)
+	{
+		pgstat_info->relation->pgstat_info = NULL;
+		pgstat_info->relation = NULL;
+	}
 
 	/* don't allow link a stats to multiple relcache entries */
-	Assert(rel->pgstat_info->relation == NULL);
+	Assert(pgstat_info->relation == NULL);
 
 	/* mark this relation as the owner */
-	rel->pgstat_info->relation = rel;
+	pgstat_info->relation = rel;
 }
 
 /*
@@ -214,14 +313,37 @@ pgstat_drop_relation(Relation rel)
 {
 	int			nest_level = GetCurrentTransactionNestLevel();
 	PgStat_TableStatus *pgstat_info;
+	bool		skip_transactional_drop = false;
 
 	/* don't track stats for relations without storage */
 	if (!RELKIND_HAS_STORAGE(rel->rd_rel->relkind))
 		return;
 
-	pgstat_drop_transactional(PGSTAT_KIND_RELATION,
-							  rel->rd_locator.dbOid,
-							  RelFileLocatorToPgStatObjid(rel->rd_locator));
+	/* Check if this drop is part of a pending rewrite */
+	if (pending_rewrites != NULL)
+	{
+		PgStat_PendingRewrite *rewrite;
+
+		for (rewrite = pending_rewrites; rewrite != NULL; rewrite = rewrite->next)
+		{
+			if (rel->rd_locator.dbOid == rewrite->old_locator.dbOid &&
+				rel->rd_locator.spcOid == rewrite->old_locator.spcOid &&
+				rel->rd_locator.relNumber == rewrite->old_locator.relNumber)
+			{
+				skip_transactional_drop = true;
+				break;
+			}
+		}
+	}
+
+	/*
+	 * If it is part of a rewrite, drop its stats later, for example in
+	 * AtEOXact_PgStat_Relations(), so skip it here.
+	 */
+	if (!skip_transactional_drop)
+		pgstat_drop_transactional(PGSTAT_KIND_RELATION,
+								  rel->rd_locator.dbOid,
+								  RelFileLocatorToPgStatObjid(rel->rd_locator));
 
 	if (!pgstat_should_count_relation(rel))
 		return;
@@ -666,6 +788,48 @@ AtEOXact_PgStat_Relations(PgStat_SubXactStatus *xact_state, bool isCommit)
 		}
 		tabstat->trans = NULL;
 	}
+
+	/* preserve the stats in case of rewrite */
+	if (isCommit && pending_rewrites != NULL)
+	{
+		PgStat_PendingRewrite *rewrite;
+		PgStat_PendingRewrite *prev = NULL;
+		PgStat_PendingRewrite *current = pending_rewrites;
+		PgStat_PendingRewrite *next;
+
+		/* reverse the rewrites list to process in chronological order */
+		while (current != NULL)
+		{
+			next = current->next;
+			current->next = prev;
+			prev = current;
+			current = next;
+		}
+
+		/* now process rewrites in chronological order */
+		for (rewrite = prev; rewrite != NULL; rewrite = rewrite->next)
+		{
+			PgStat_EntryRef *old_entry_ref;
+
+			old_entry_ref = pgstat_fetch_pending_entry(PGSTAT_KIND_RELATION,
+													   rewrite->old_locator.dbOid,
+													   RelFileLocatorToPgStatObjid(rewrite->old_locator));
+
+			if (old_entry_ref && old_entry_ref->pending)
+				pgstat_relation_flush_cb(old_entry_ref, false);
+
+			pgstat_copy_relation_stats(rewrite->new_locator,
+									   rewrite->old_locator, true);
+
+			/* drop old locator's stats */
+			if (!pgstat_drop_entry(PGSTAT_KIND_RELATION,
+								   rewrite->old_locator.dbOid,
+								   RelFileLocatorToPgStatObjid(rewrite->old_locator)))
+				pgstat_request_entry_refs_gc();
+		}
+	}
+
+	pending_rewrites = NULL;
 }
 
 /*
@@ -681,6 +845,30 @@ AtEOSubXact_PgStat_Relations(PgStat_SubXactStatus *xact_state, bool isCommit, in
 	PgStat_TableXactStatus *trans;
 	PgStat_TableXactStatus *next_trans;
 
+	/*
+	 * If we don't commit then remove the associated rewrites if any, to keep
+	 * the rewrite chain in sync with what will be eventually committed.
+	 */
+	if (!isCommit)
+	{
+		PgStat_PendingRewrite **rewrite_ptr = &pending_rewrites;
+
+		while (*rewrite_ptr != NULL)
+		{
+			if ((*rewrite_ptr)->nest_level >= nestDepth)
+			{
+				PgStat_PendingRewrite *to_remove = *rewrite_ptr;
+
+				*rewrite_ptr = (*rewrite_ptr)->next;
+				pfree(to_remove);
+			}
+			else
+			{
+				rewrite_ptr = &((*rewrite_ptr)->next);
+			}
+		}
+	}
+
 	for (trans = xact_state->first; trans != NULL; trans = next_trans)
 	{
 		PgStat_TableStatus *tabstat;
@@ -760,11 +948,19 @@ void
 AtPrepare_PgStat_Relations(PgStat_SubXactStatus *xact_state)
 {
 	PgStat_TableXactStatus *trans;
+	PgStat_PendingRewrite *rewrite;
 
+	/*
+	 * For each tabstat, find its matching rewrite and remove it from the
+	 * pending rewrites list. This way, after processing all tabstats, pending
+	 * rewrites will only contain rewrite only transactions.
+	 */
 	for (trans = xact_state->first; trans != NULL; trans = trans->next)
 	{
 		PgStat_TableStatus *tabstat PG_USED_FOR_ASSERTS_ONLY;
 		TwoPhasePgStatRecord record;
+		PgStat_PendingRewrite **rewrite_ptr;
+		bool		found_rewrite = false;
 
 		Assert(trans->nest_level == 1);
 		Assert(trans->upper == NULL);
@@ -784,10 +980,83 @@ AtPrepare_PgStat_Relations(PgStat_SubXactStatus *xact_state)
 			record.locator = tabstat->locator;
 
 		record.truncdropped = trans->truncdropped;
+		record.rewrite_nest_level = 0;
+
+		/*
+		 * Look for a matching rewrite and remove it from pending rewrites. We
+		 * check three possible matches:
+		 *
+		 * The new_locator when stats have been added after the rewrite. The
+		 * old_locator when stats have been added before the rewrite but not
+		 * after. The original_locator when this tabstat is part of a rewrite
+		 * chain.
+		 */
+		rewrite_ptr = &pending_rewrites;
+		while (*rewrite_ptr != NULL)
+		{
+			rewrite = *rewrite_ptr;
+
+			if ((record.locator.dbOid == rewrite->new_locator.dbOid &&
+				 record.locator.spcOid == rewrite->new_locator.spcOid &&
+				 record.locator.relNumber == rewrite->new_locator.relNumber) ||
+				(tabstat->locator.dbOid == rewrite->old_locator.dbOid &&
+				 tabstat->locator.spcOid == rewrite->old_locator.spcOid &&
+				 tabstat->locator.relNumber == rewrite->old_locator.relNumber) ||
+				(tabstat->locator.dbOid == rewrite->original_locator.dbOid &&
+				 tabstat->locator.spcOid == rewrite->original_locator.spcOid &&
+				 tabstat->locator.relNumber == rewrite->original_locator.relNumber))
+			{
+				/*
+				 * Found matching rewrite. Record the rewrite information and
+				 * remove this rewrite from the list since it's now handled.
+				 */
+				record.rewrite_old_locator = rewrite->original_locator;
+				record.rewrite_nest_level = rewrite->nest_level;
+				record.locator = rewrite->new_locator;
+				found_rewrite = true;
+
+				/* Remove from pending_rewrites list */
+				*rewrite_ptr = rewrite->next;
+				pfree(rewrite);
+				break;
+			}
+			else
+			{
+				/* Move to next rewrite in the list */
+				rewrite_ptr = &(rewrite->next);
+			}
+		}
+
+		/* If no rewrite found, clear the rewrite fields */
+		if (!found_rewrite)
+		{
+			memset(&record.rewrite_old_locator, 0, sizeof(RelFileLocator));
+		}
+
+		RegisterTwoPhaseRecord(TWOPHASE_RM_PGSTAT_ID, 0,
+							   &record, sizeof(TwoPhasePgStatRecord));
+	}
+
+	/*
+	 * Now process any rewrites still pending. These are rewrite only
+	 * transactions. We need to preserve their stats even though there's no
+	 * tabstat entry for them.
+	 */
+	for (rewrite = pending_rewrites; rewrite != NULL; rewrite = rewrite->next)
+	{
+		TwoPhasePgStatRecord record;
+
+		memset(&record, 0, sizeof(TwoPhasePgStatRecord));
+		record.locator = rewrite->new_locator;
+		record.rewrite_old_locator = rewrite->original_locator;
+		record.rewrite_nest_level = rewrite->nest_level;
+		record.truncdropped = false;
 
 		RegisterTwoPhaseRecord(TWOPHASE_RM_PGSTAT_ID, 0,
 							   &record, sizeof(TwoPhasePgStatRecord));
 	}
+
+	pending_rewrites = NULL;
 }
 
 /*
@@ -810,6 +1079,8 @@ PostPrepare_PgStat_Relations(PgStat_SubXactStatus *xact_state)
 		tabstat = trans->parent;
 		tabstat->trans = NULL;
 	}
+
+	pending_rewrites = NULL;
 }
 
 /*
@@ -845,6 +1116,29 @@ pgstat_twophase_postcommit(FullTransactionId fxid, uint16 info,
 	pgstat_info->counts.changed_tuples +=
 		rec->tuples_inserted + rec->tuples_updated +
 		rec->tuples_deleted;
+
+	if (rec->rewrite_nest_level > 0)
+	{
+		PgStat_EntryRef *old_entry_ref;
+
+		/* Flush any pending stats for old locator first */
+		old_entry_ref = pgstat_fetch_pending_entry(PGSTAT_KIND_RELATION,
+												   rec->rewrite_old_locator.dbOid,
+												   RelFileLocatorToPgStatObjid(rec->rewrite_old_locator));
+
+		if (old_entry_ref && old_entry_ref->pending)
+			pgstat_relation_flush_cb(old_entry_ref, false);
+
+		/* Copy stats from old to new locator */
+		pgstat_copy_relation_stats(rec->locator, rec->rewrite_old_locator,
+								   true);
+
+		/* Drop old locator's stats */
+		if (!pgstat_drop_entry(PGSTAT_KIND_RELATION,
+							   rec->rewrite_old_locator.dbOid,
+							   RelFileLocatorToPgStatObjid(rec->rewrite_old_locator)))
+			pgstat_request_entry_refs_gc();
+	}
 }
 
 /*
@@ -859,9 +1153,26 @@ pgstat_twophase_postabort(FullTransactionId fxid, uint16 info,
 {
 	TwoPhasePgStatRecord *rec = (TwoPhasePgStatRecord *) recdata;
 	PgStat_TableStatus *pgstat_info;
+	RelFileLocator target_locator;
+
+	/*
+	 * For aborted transactions with rewrites (like TRUNCATE), we need to
+	 * restore stats to the old locator, not the new one. The new locator
+	 * should be dropped since the rewrite is being rolled back.
+	 */
+	if (rec->rewrite_nest_level > 0)
+	{
+		/* Use the old locator */
+		target_locator = rec->rewrite_old_locator;
+	}
+	else
+	{
+		/* No rewrite, use the original locator */
+		target_locator = rec->locator;
+	}
 
 	/* Find or create a tabstat entry for the target locator */
-	pgstat_info = pgstat_prep_relation_pending(rec->locator);
+	pgstat_info = pgstat_prep_relation_pending(target_locator);
 
 	/* Same math as in AtEOXact_PgStat, abort case */
 	if (rec->truncdropped)
@@ -916,7 +1227,17 @@ pgstat_relation_flush_cb(PgStat_EntryRef *entry_ref, bool nowait)
 	tabentry->numscans += lstats->counts.numscans;
 	if (lstats->counts.numscans)
 	{
-		TimestampTz t = GetCurrentTransactionStopTimestamp();
+		TimestampTz t;
+
+		/*
+		 * Checking the transaction state due to the flush call in
+		 * pgstat_twophase_postcommit() that would break the assertion on the
+		 * state in GetCurrentTransactionStopTimestamp().
+		 */
+		if (!IsTransactionState())
+			t = GetCurrentTransactionStopTimestamp();
+		else
+			t = GetCurrentTimestamp();
 
 		if (t > tabentry->lastscan)
 			tabentry->lastscan = t;
@@ -1167,3 +1488,45 @@ pgstat_reloid_to_relfilelocator(Oid reloid, RelFileLocator *locator)
 	ReleaseSysCache(tuple);
 	return result;
 }
+
+/*
+ * Mark that a relation rewrite has occurred, preserving the original locator
+ * so stats can be copied at transaction commit.
+ */
+void
+pgstat_mark_rewrite(RelFileLocator old_locator, RelFileLocator new_locator)
+{
+	PgStat_PendingRewrite *rewrite;
+	PgStat_PendingRewrite *existing;
+	RelFileLocator original_locator = old_locator;
+
+	for (existing = pending_rewrites; existing != NULL; existing = existing->next)
+	{
+		if (old_locator.dbOid == existing->new_locator.dbOid &&
+			old_locator.spcOid == existing->new_locator.spcOid &&
+			old_locator.relNumber == existing->new_locator.relNumber)
+		{
+			original_locator = existing->original_locator;
+			break;
+		}
+	}
+
+	/* Allocate in TopTransactionContext memory context */
+	rewrite = MemoryContextAlloc(TopTransactionContext,
+								 sizeof(PgStat_PendingRewrite));
+
+	rewrite->old_locator = old_locator;
+	rewrite->new_locator = new_locator;
+	rewrite->original_locator = original_locator;
+	rewrite->nest_level = GetCurrentTransactionNestLevel();
+
+	/* Add to the list */
+	rewrite->next = pending_rewrites;
+	pending_rewrites = rewrite;
+}
+
+void
+pgstat_clear_rewrite(void)
+{
+	pending_rewrites = NULL;
+}
diff --git a/src/backend/utils/activity/pgstat_xact.c b/src/backend/utils/activity/pgstat_xact.c
index bc9864bd8d9..f8cf3755ce2 100644
--- a/src/backend/utils/activity/pgstat_xact.c
+++ b/src/backend/utils/activity/pgstat_xact.c
@@ -55,6 +55,8 @@ AtEOXact_PgStat(bool isCommit, bool parallel)
 	}
 	pgStatXactStack = NULL;
 
+	pgstat_clear_rewrite();
+
 	/* Make sure any stats snapshot is thrown away */
 	pgstat_clear_snapshot();
 }
@@ -360,8 +362,29 @@ create_drop_transactional_internal(PgStat_Kind kind, Oid dboid, uint64 objid, bo
 void
 pgstat_create_transactional(PgStat_Kind kind, Oid dboid, uint64 objid)
 {
-	if (pgstat_get_entry_ref(kind, dboid, objid, false, NULL))
+	PgStat_EntryRef *entry_ref;
+
+	entry_ref = pgstat_get_entry_ref(kind, dboid, objid, false, NULL);
+
+	if (entry_ref)
 	{
+		/*
+		 * For relations stats, we key by physical file location, not by
+		 * relation OID. This means during operations like ALTER TYPE where
+		 * the relation OID changes but the relfilenode stays the same (no
+		 * actual rewrite needed), we'll find an existing entry.
+		 *
+		 * This is expected behavior, we want to preserve stats across the
+		 * catalog change. Simply reset and recreate the entry for the new
+		 * relation OID without warning.
+		 */
+		if (kind == PGSTAT_KIND_RELATION)
+		{
+			pgstat_reset(kind, dboid, objid);
+			create_drop_transactional_internal(kind, dboid, objid, true);
+			return;
+		}
+
 		ereport(WARNING,
 				errmsg("resetting existing statistics for kind %s, db=%u, oid=%" PRIu64,
 					   (pgstat_get_kind_info(kind))->name, dboid,
diff --git a/src/backend/utils/cache/relcache.c b/src/backend/utils/cache/relcache.c
index 2d0cb7bcfd4..c98e5c51d63 100644
--- a/src/backend/utils/cache/relcache.c
+++ b/src/backend/utils/cache/relcache.c
@@ -85,6 +85,7 @@
 #include "utils/inval.h"
 #include "utils/lsyscache.h"
 #include "utils/memutils.h"
+#include "utils/pgstat_internal.h"
 #include "utils/relmapper.h"
 #include "utils/resowner.h"
 #include "utils/snapmgr.h"
@@ -3780,6 +3781,7 @@ RelationSetNewRelfilenumber(Relation relation, char persistence)
 	MultiXactId minmulti = InvalidMultiXactId;
 	TransactionId freezeXid = InvalidTransactionId;
 	RelFileLocator newrlocator;
+	RelFileLocator oldrlocator = relation->rd_locator;
 
 	if (!IsBinaryUpgrade)
 	{
@@ -3951,6 +3953,10 @@ RelationSetNewRelfilenumber(Relation relation, char persistence)
 
 	table_close(pg_class, RowExclusiveLock);
 
+	/* Mark that a rewrite happened */
+	if (RELKIND_HAS_STORAGE(relation->rd_rel->relkind))
+		pgstat_mark_rewrite(oldrlocator, newrlocator);
+
 	/*
 	 * Make the pg_class row change or relation map change visible.  This will
 	 * cause the relcache entry to get updated, too.
diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index 3102f86aa24..8750c025bbe 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -670,7 +670,7 @@ extern PgStat_FunctionCounts *find_funcstat_entry(Oid func_id);
 
 extern void pgstat_create_relation(Relation rel);
 extern void pgstat_drop_relation(Relation rel);
-extern void pgstat_copy_relation_stats(Relation dst, Relation src);
+extern void pgstat_copy_relation_stats(RelFileLocator dst, RelFileLocator src, bool increment);
 
 extern void pgstat_init_relation(Relation rel);
 extern void pgstat_assoc_relation(Relation rel);
@@ -682,6 +682,9 @@ extern void pgstat_report_vacuum(Relation rel, PgStat_Counter livetuples,
 extern void pgstat_report_analyze(Relation rel,
 								  PgStat_Counter livetuples, PgStat_Counter deadtuples,
 								  bool resetcounter, TimestampTz starttime);
+extern void pgstat_mark_rewrite(RelFileLocator old_locator,
+								RelFileLocator new_locator);
+extern void pgstat_clear_rewrite(void);
 
 /*
  * If stats are enabled, but pending data hasn't been prepared yet, call
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index 3451538565e..0ca3eae8026 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -2268,6 +2268,7 @@ PgStat_KindInfo
 PgStat_LocalState
 PgStat_PendingDroppedStatsItem
 PgStat_PendingIO
+PgStat_PendingRewrite
 PgStat_SLRUStats
 PgStat_ShmemControl
 PgStat_Snapshot
-- 
2.34.1


--tV2Nti+lXhoeT6I5--





^ permalink  raw  reply  [nested|flat] 333+ 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; 333+ 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] 333+ 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; 333+ 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] 333+ 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; 333+ 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] 333+ 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; 333+ 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] 333+ 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; 333+ 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] 333+ 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; 333+ 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] 333+ 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; 333+ 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] 333+ 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; 333+ 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] 333+ 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; 333+ 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] 333+ 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; 333+ 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] 333+ 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; 333+ 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] 333+ 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; 333+ 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] 333+ 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; 333+ 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] 333+ 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; 333+ 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] 333+ 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; 333+ 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] 333+ 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; 333+ 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] 333+ 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; 333+ 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] 333+ 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; 333+ 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] 333+ 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; 333+ 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--





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