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.

This largely reverts commit 133924e13.  That's safe to do now
because bf6c614a2 decoupled the evaluation context used by
TupleHashTableMatch from that used for hash function evaluation,
so that there's no longer a risk of resetting the innerecontext
too soon.

Per discussion of bug #19040, although this is not directly
a fix for that.

Author: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Haiyang Li <mohen.lhy@alibaba-inc.com>
Reviewed-by: Fei Changhong <feichanghong@qq.com>
Discussion: https://postgr.es/m/19040-c9b6073ef814f48c@postgresql.org

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);
 

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: */