v04-0002-Eliminate-duplicative-hashtempcxt-in-nodeSubplan.patch

From 3df0846dfe41955353287b4b33caf43e1dd2e399 Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Mon, 8 Sep 2025 16:41:03 -0400
Subject: [PATCH v04 2/2] Eliminate duplicative hashtempcxt in nodeSubplan.c.

Instead of building a separate memory context that's used just
for running hash functions, make the hash functions run in the
per-tuple context of the node's innerecontext.  This saves a
little space at runtime, and it avoids needing to reset two
contexts instead of one inside buildSubPlanHash's main loop.
Per discussion of bug #19040, although this is not directly
a fix for that.

Author: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/19040-c9b6073ef814f48c@postgresql.org
---
 src/backend/executor/nodeSubplan.c | 19 +++++--------------
 src/include/nodes/execnodes.h      |  1 -
 2 files changed, 5 insertions(+), 15 deletions(-)

diff --git a/src/backend/executor/nodeSubplan.c b/src/backend/executor/nodeSubplan.c
index 8e55dcc159b..53fb56f7388 100644
--- a/src/backend/executor/nodeSubplan.c
+++ b/src/backend/executor/nodeSubplan.c
@@ -191,8 +191,8 @@ ExecHashSubPlan(SubPlanState *node,
 	 */
 	ExecClearTuple(slot);
 
-	/* Also must reset the hashtempcxt after each hashtable lookup. */
-	MemoryContextReset(node->hashtempcxt);
+	/* Also must reset the innerecontext after each hashtable lookup. */
+	ResetExprContext(node->innerecontext);
 
 	return BoolGetDatum(result);
 }
@@ -529,7 +529,7 @@ buildSubPlanHash(SubPlanState *node, ExprContext *econtext)
 											  0,
 											  node->planstate->state->es_query_cxt,
 											  node->hashtablecxt,
-											  node->hashtempcxt,
+											  innerecontext->ecxt_per_tuple_memory,
 											  false);
 
 	if (!subplan->unknownEqFalse)
@@ -558,7 +558,7 @@ buildSubPlanHash(SubPlanState *node, ExprContext *econtext)
 												  0,
 												  node->planstate->state->es_query_cxt,
 												  node->hashtablecxt,
-												  node->hashtempcxt,
+												  innerecontext->ecxt_per_tuple_memory,
 												  false);
 	}
 	else
@@ -620,12 +620,9 @@ buildSubPlanHash(SubPlanState *node, ExprContext *econtext)
 
 		/*
 		 * Reset innerecontext after each inner tuple to free any memory used
-		 * during ExecProject.
+		 * during ExecProject and hashtable lookup.
 		 */
 		ResetExprContext(innerecontext);
-
-		/* Also must reset the hashtempcxt after each hashtable lookup. */
-		MemoryContextReset(node->hashtempcxt);
 	}
 
 	/*
@@ -842,7 +839,6 @@ ExecInitSubPlan(SubPlan *subplan, PlanState *parent)
 	sstate->hashtable = NULL;
 	sstate->hashnulls = NULL;
 	sstate->hashtablecxt = NULL;
-	sstate->hashtempcxt = NULL;
 	sstate->innerecontext = NULL;
 	sstate->keyColIdx = NULL;
 	sstate->tab_eq_funcoids = NULL;
@@ -898,11 +894,6 @@ ExecInitSubPlan(SubPlan *subplan, PlanState *parent)
 			AllocSetContextCreate(CurrentMemoryContext,
 								  "Subplan HashTable Context",
 								  ALLOCSET_DEFAULT_SIZES);
-		/* and a small one for the hash tables to use as temp storage */
-		sstate->hashtempcxt =
-			AllocSetContextCreate(CurrentMemoryContext,
-								  "Subplan HashTable Temp Context",
-								  ALLOCSET_SMALL_SIZES);
 		/* and a short-lived exprcontext for function evaluation */
 		sstate->innerecontext = CreateExprContext(estate);
 
diff --git a/src/include/nodes/execnodes.h b/src/include/nodes/execnodes.h
index de782014b2d..71857feae48 100644
--- a/src/include/nodes/execnodes.h
+++ b/src/include/nodes/execnodes.h
@@ -1020,7 +1020,6 @@ typedef struct SubPlanState
 	bool		havehashrows;	/* true if hashtable is not empty */
 	bool		havenullrows;	/* true if hashnulls is not empty */
 	MemoryContext hashtablecxt; /* memory context containing hash tables */
-	MemoryContext hashtempcxt;	/* temp memory context for hash tables */
 	ExprContext *innerecontext; /* econtext for computing inner tuples */
 	int			numCols;		/* number of columns being hashed */
 	/* each of the remaining fields is an array of length numCols: */
-- 
2.43.7