Fix improper repetition of previous results from a hashed aggregate.

ExecReScanAgg's check for whether it could re-use a previously calculated
hashtable neglected the possibility that the Agg node might reference
PARAM_EXEC Params that are not referenced by its input plan node.  That's
okay if the Params are in upper tlist or qual expressions; but if one
appears in aggregate input expressions, then the hashtable contents need
to be recomputed when the Param's value changes.

To avoid unnecessary performance degradation in the case of a Param that
isn't within an aggregate input, add logic to the planner to determine
which Params are within aggregate inputs.  This requires a new field in
struct Agg, but fortunately we never write plans to disk, so this isn't
an initdb-forcing change.

Per report from Jeevan Chalke.  This has been broken since forever,
so back-patch to all supported branches.

Andrew Gierth, with minor adjustments by me

Report: <CAM2+6=VY8ykfLT5Q8vb9B6EbeBk-NGuLbT6seaQ+Fq4zXvrDcA@mail.gmail.com>

1 files changed, 9 insertions, 6 deletions

diff --git a/src/backend/executor/nodeAgg.c b/src/backend/executor/nodeAgg.c
index 34afb24b380..b4b6699dcdf 100644
--- a/src/backend/executor/nodeAgg.c
+++ b/src/backend/executor/nodeAgg.c
@@ -1905,13 +1905,14 @@ void
 ExecReScanAgg(AggState *node)
 {
 	ExprContext *econtext = node->ss.ps.ps_ExprContext;
+	Agg		   *aggnode = (Agg *) node->ss.ps.plan;
 	int			aggno;
 
 	node->agg_done = false;
 
 	node->ss.ps.ps_TupFromTlist = false;
 
-	if (((Agg *) node->ss.ps.plan)->aggstrategy == AGG_HASHED)
+	if (aggnode->aggstrategy == AGG_HASHED)
 	{
 		/*
 		 * In the hashed case, if we haven't yet built the hash table then we
@@ -1923,11 +1924,13 @@ ExecReScanAgg(AggState *node)
 			return;
 
 		/*
-		 * If we do have the hash table and the subplan does not have any
-		 * parameter changes, then we can just rescan the existing hash table;
-		 * no need to build it again.
+		 * If we do have the hash table, and the subplan does not have any
+		 * parameter changes, and none of our own parameter changes affect
+		 * input expressions of the aggregated functions, then we can just
+		 * rescan the existing hash table; no need to build it again.
 		 */
-		if (node->ss.ps.lefttree->chgParam == NULL)
+		if (node->ss.ps.lefttree->chgParam == NULL &&
+			!bms_overlap(node->ss.ps.chgParam, aggnode->aggParams))
 		{
 			ResetTupleHashIterator(node->hashtable, &node->hashiter);
 			return;
@@ -1964,7 +1967,7 @@ ExecReScanAgg(AggState *node)
 	 */
 	MemoryContextResetAndDeleteChildren(node->aggcontext);
 
-	if (((Agg *) node->ss.ps.plan)->aggstrategy == AGG_HASHED)
+	if (aggnode->aggstrategy == AGG_HASHED)
 	{
 		/* Rebuild an empty hash table */
 		build_hash_table(node);