[Peter Eisentraut <peter.eisentraut@2ndquadrant.com>]

When updating a table row with generated columns, we only need to 
recompute those generated columns whose base columns have changed in 
this update and keep the rest unchanged.  This can result in a 
significant performance benefit (easy to reproduce for example with a 
tsvector column).  The required information was already kept in 
RangeTblEntry.extraUpdatedCols; we just have to make use of it.

A small problem is that right now ExecSimpleRelationUpdate() does not 
populate extraUpdatedCols.  That needs fixing first.  This is also 
related to the issue discussed in "logical replication does not fire 
per-column triggers"[0].  I'll leave my patch here while that issue is 
being resolved.


[0]: 
https://www.postgresql.org/message-id/flat/21673e2d-597c-6afe-637e-e8b10425b240%402ndquadrant.com

-- 
Peter Eisentraut              http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

Attachments:

0001-Optimize-update-of-tables-with-generated-columns.patch (text/plain, 7.4K) download
From 3b80f3147c0e983a2e8ad41be7df54c6480c0d8f Mon Sep 17 00:00:00 2001
From: Peter Eisentraut <peter@eisentraut.org>
Date: Fri, 20 Dec 2019 22:52:31 +0100
Subject: [PATCH] Optimize update of tables with generated columns

When updating a table row with generated columns, only recompute those
generated columns whose base columns have changed in this update and
keep the rest unchanged.  This can result in a significant performance
benefit.  The required information was already kept in
RangeTblEntry.extraUpdatedCols; we just have to make use of it.

FIXME: ExecSimpleRelationUpdate() does not currently populate
extraUpdatedCols.  That needs fixing first.
---
 src/backend/commands/copy.c            |  2 +-
 src/backend/executor/execReplication.c |  4 +--
 src/backend/executor/nodeModifyTable.c | 37 +++++++++++++++++++++-----
 src/include/executor/nodeModifyTable.h |  2 +-
 src/include/nodes/execnodes.h          |  3 +++
 5 files changed, 38 insertions(+), 10 deletions(-)

diff --git a/src/backend/commands/copy.c b/src/backend/commands/copy.c
index 42a147b67d..a0758cd39c 100644
--- a/src/backend/commands/copy.c
+++ b/src/backend/commands/copy.c
@@ -3221,7 +3221,7 @@ CopyFrom(CopyState cstate)
 				/* Compute stored generated columns */
 				if (resultRelInfo->ri_RelationDesc->rd_att->constr &&
 					resultRelInfo->ri_RelationDesc->rd_att->constr->has_generated_stored)
-					ExecComputeStoredGenerated(estate, myslot);
+					ExecComputeStoredGenerated(estate, myslot, CMD_INSERT);
 
 				/*
 				 * If the target is a plain table, check the constraints of
diff --git a/src/backend/executor/execReplication.c b/src/backend/executor/execReplication.c
index 95e027c970..8cbec36b18 100644
--- a/src/backend/executor/execReplication.c
+++ b/src/backend/executor/execReplication.c
@@ -416,7 +416,7 @@ ExecSimpleRelationInsert(EState *estate, TupleTableSlot *slot)
 		/* Compute stored generated columns */
 		if (rel->rd_att->constr &&
 			rel->rd_att->constr->has_generated_stored)
-			ExecComputeStoredGenerated(estate, slot);
+			ExecComputeStoredGenerated(estate, slot, CMD_INSERT);
 
 		/* Check the constraints of the tuple */
 		if (rel->rd_att->constr)
@@ -482,7 +482,7 @@ ExecSimpleRelationUpdate(EState *estate, EPQState *epqstate,
 		/* Compute stored generated columns */
 		if (rel->rd_att->constr &&
 			rel->rd_att->constr->has_generated_stored)
-			ExecComputeStoredGenerated(estate, slot);
+			ExecComputeStoredGenerated(estate, slot, CMD_UPDATE);
 
 		/* Check the constraints of the tuple */
 		if (rel->rd_att->constr)
diff --git a/src/backend/executor/nodeModifyTable.c b/src/backend/executor/nodeModifyTable.c
index 9ba1d78344..5878041ee6 100644
--- a/src/backend/executor/nodeModifyTable.c
+++ b/src/backend/executor/nodeModifyTable.c
@@ -246,7 +246,7 @@ ExecCheckTIDVisible(EState *estate,
  * Compute stored generated columns for a tuple
  */
 void
-ExecComputeStoredGenerated(EState *estate, TupleTableSlot *slot)
+ExecComputeStoredGenerated(EState *estate, TupleTableSlot *slot, CmdType cmdtype)
 {
 	ResultRelInfo *resultRelInfo = estate->es_result_relation_info;
 	Relation	rel = resultRelInfo->ri_RelationDesc;
@@ -269,6 +269,7 @@ ExecComputeStoredGenerated(EState *estate, TupleTableSlot *slot)
 
 		resultRelInfo->ri_GeneratedExprs =
 			(ExprState **) palloc(natts * sizeof(ExprState *));
+		resultRelInfo->ri_NumGeneratedNeeded = 0;
 
 		for (int i = 0; i < natts; i++)
 		{
@@ -276,18 +277,41 @@ ExecComputeStoredGenerated(EState *estate, TupleTableSlot *slot)
 			{
 				Expr	   *expr;
 
+				/*
+				 * If it's an update and the current column was not marked as
+				 * being updated, then we can skip the computation.  But if
+				 * there is a BEFORE ROW UPDATE trigger, we cannot skip
+				 * because the trigger might affect additional columns.
+				 */
+				if (cmdtype == CMD_UPDATE &&
+					!(rel->trigdesc && rel->trigdesc->trig_update_before_row) &&
+					!bms_is_member(i + 1 - FirstLowInvalidHeapAttributeNumber,
+								   exec_rt_fetch(resultRelInfo->ri_RangeTableIndex, estate)->extraUpdatedCols))
+				{
+					resultRelInfo->ri_GeneratedExprs[i] = NULL;
+					continue;
+				}
+
 				expr = (Expr *) build_column_default(rel, i + 1);
 				if (expr == NULL)
 					elog(ERROR, "no generation expression found for column number %d of table \"%s\"",
 						 i + 1, RelationGetRelationName(rel));
 
 				resultRelInfo->ri_GeneratedExprs[i] = ExecPrepareExpr(expr, estate);
+				resultRelInfo->ri_NumGeneratedNeeded++;
 			}
 		}
 
 		MemoryContextSwitchTo(oldContext);
 	}
 
+	/*
+	 * If no generated columns have been affected by this change, then skip
+	 * the rest.
+	 */
+	if (resultRelInfo->ri_NumGeneratedNeeded == 0)
+		return;
+
 	oldContext = MemoryContextSwitchTo(GetPerTupleMemoryContext(estate));
 
 	values = palloc(sizeof(*values) * natts);
@@ -300,7 +324,8 @@ ExecComputeStoredGenerated(EState *estate, TupleTableSlot *slot)
 	{
 		Form_pg_attribute attr = TupleDescAttr(tupdesc, i);
 
-		if (attr->attgenerated == ATTRIBUTE_GENERATED_STORED)
+		if (attr->attgenerated == ATTRIBUTE_GENERATED_STORED &&
+			resultRelInfo->ri_GeneratedExprs[i])
 		{
 			ExprContext *econtext;
 			Datum		val;
@@ -392,7 +417,7 @@ ExecInsert(ModifyTableState *mtstate,
 		 */
 		if (resultRelationDesc->rd_att->constr &&
 			resultRelationDesc->rd_att->constr->has_generated_stored)
-			ExecComputeStoredGenerated(estate, slot);
+			ExecComputeStoredGenerated(estate, slot, CMD_INSERT);
 
 		/*
 		 * insert into foreign table: let the FDW do it
@@ -427,7 +452,7 @@ ExecInsert(ModifyTableState *mtstate,
 		 */
 		if (resultRelationDesc->rd_att->constr &&
 			resultRelationDesc->rd_att->constr->has_generated_stored)
-			ExecComputeStoredGenerated(estate, slot);
+			ExecComputeStoredGenerated(estate, slot, CMD_INSERT);
 
 		/*
 		 * Check any RLS WITH CHECK policies.
@@ -1088,7 +1113,7 @@ ExecUpdate(ModifyTableState *mtstate,
 		 */
 		if (resultRelationDesc->rd_att->constr &&
 			resultRelationDesc->rd_att->constr->has_generated_stored)
-			ExecComputeStoredGenerated(estate, slot);
+			ExecComputeStoredGenerated(estate, slot, CMD_UPDATE);
 
 		/*
 		 * update in foreign table: let the FDW do it
@@ -1125,7 +1150,7 @@ ExecUpdate(ModifyTableState *mtstate,
 		 */
 		if (resultRelationDesc->rd_att->constr &&
 			resultRelationDesc->rd_att->constr->has_generated_stored)
-			ExecComputeStoredGenerated(estate, slot);
+			ExecComputeStoredGenerated(estate, slot, CMD_UPDATE);
 
 		/*
 		 * Check any RLS UPDATE WITH CHECK policies
diff --git a/src/include/executor/nodeModifyTable.h b/src/include/executor/nodeModifyTable.h
index 891b119608..d44a64cb49 100644
--- a/src/include/executor/nodeModifyTable.h
+++ b/src/include/executor/nodeModifyTable.h
@@ -15,7 +15,7 @@
 
 #include "nodes/execnodes.h"
 
-extern void ExecComputeStoredGenerated(EState *estate, TupleTableSlot *slot);
+extern void ExecComputeStoredGenerated(EState *estate, TupleTableSlot *slot, CmdType cmdtype);
 
 extern ModifyTableState *ExecInitModifyTable(ModifyTable *node, EState *estate, int eflags);
 extern void ExecEndModifyTable(ModifyTableState *node);
diff --git a/src/include/nodes/execnodes.h b/src/include/nodes/execnodes.h
index 0c2a77aaf8..93cca2e08d 100644
--- a/src/include/nodes/execnodes.h
+++ b/src/include/nodes/execnodes.h
@@ -456,6 +456,9 @@ typedef struct ResultRelInfo
 	/* array of stored generated columns expr states */
 	ExprState **ri_GeneratedExprs;
 
+	/* number of stored generated columns we need to compute */
+	int			ri_NumGeneratedNeeded;
+
 	/* for removing junk attributes from tuples */
 	JunkFilter *ri_junkFilter;
 
-- 
2.24.1