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>

5 files changed, 58 insertions, 7 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);
diff --git a/src/backend/nodes/copyfuncs.c b/src/backend/nodes/copyfuncs.c
index eb613aec7af..5ed00f68a28 100644
--- a/src/backend/nodes/copyfuncs.c
+++ b/src/backend/nodes/copyfuncs.c
@@ -769,6 +769,7 @@ _copyAgg(Agg *from)
 		COPY_POINTER_FIELD(grpOperators, from->numCols * sizeof(Oid));
 	}
 	COPY_SCALAR_FIELD(numGroups);
+	COPY_BITMAPSET_FIELD(aggParams);
 
 	return newnode;
 }
diff --git a/src/backend/nodes/outfuncs.c b/src/backend/nodes/outfuncs.c
index 01b2d6d2d4a..55d4d9b74b9 100644
--- a/src/backend/nodes/outfuncs.c
+++ b/src/backend/nodes/outfuncs.c
@@ -642,6 +642,7 @@ _outAgg(StringInfo str, Agg *node)
 		appendStringInfo(str, " %u", node->grpOperators[i]);
 
 	WRITE_LONG_FIELD(numGroups);
+	WRITE_BITMAPSET_FIELD(aggParams);
 }
 
 static void
diff --git a/src/backend/optimizer/plan/createplan.c b/src/backend/optimizer/plan/createplan.c
index 77cf33d1f79..339f5156352 100644
--- a/src/backend/optimizer/plan/createplan.c
+++ b/src/backend/optimizer/plan/createplan.c
@@ -3940,6 +3940,7 @@ make_agg(PlannerInfo *root, List *tlist, List *qual,
 	node->grpColIdx = grpColIdx;
 	node->grpOperators = grpOperators;
 	node->numGroups = numGroups;
+	node->aggParams = NULL;		/* SS_finalize_plan() will fill this */
 
 	copy_plan_costsize(plan, lefttree); /* only care about copying size */
 	cost_agg(&agg_path, root,
diff --git a/src/backend/optimizer/plan/subselect.c b/src/backend/optimizer/plan/subselect.c
index c50709ccc1b..7890305a94e 100644
--- a/src/backend/optimizer/plan/subselect.c
+++ b/src/backend/optimizer/plan/subselect.c
@@ -81,6 +81,7 @@ static Bitmapset *finalize_plan(PlannerInfo *root,
 			  Bitmapset *valid_params,
 			  Bitmapset *scan_params);
 static bool finalize_primnode(Node *node, finalize_primnode_context *context);
+static bool finalize_agg_primnode(Node *node, finalize_primnode_context *context);
 
 
 /*
@@ -2351,6 +2352,29 @@ finalize_plan(PlannerInfo *root, Plan *plan, Bitmapset *valid_params,
 										 locally_added_param);
 			break;
 
+		case T_Agg:
+			{
+				Agg		   *agg = (Agg *) plan;
+
+				/*
+				 * AGG_HASHED plans need to know which Params are referenced
+				 * in aggregate calls.  Do a separate scan to identify them.
+				 */
+				if (agg->aggstrategy == AGG_HASHED)
+				{
+					finalize_primnode_context aggcontext;
+
+					aggcontext.root = root;
+					aggcontext.paramids = NULL;
+					finalize_agg_primnode((Node *) agg->plan.targetlist,
+										  &aggcontext);
+					finalize_agg_primnode((Node *) agg->plan.qual,
+										  &aggcontext);
+					agg->aggParams = aggcontext.paramids;
+				}
+			}
+			break;
+
 		case T_WindowAgg:
 			finalize_primnode(((WindowAgg *) plan)->startOffset,
 							  &context);
@@ -2359,7 +2383,6 @@ finalize_plan(PlannerInfo *root, Plan *plan, Bitmapset *valid_params,
 			break;
 
 		case T_Hash:
-		case T_Agg:
 		case T_Material:
 		case T_Sort:
 		case T_Unique:
@@ -2504,6 +2527,28 @@ finalize_primnode(Node *node, finalize_primnode_context *context)
 }
 
 /*
+ * finalize_agg_primnode: find all Aggref nodes in the given expression tree,
+ * and add IDs of all PARAM_EXEC params appearing within their aggregated
+ * arguments to the result set.
+ */
+static bool
+finalize_agg_primnode(Node *node, finalize_primnode_context *context)
+{
+	if (node == NULL)
+		return false;
+	if (IsA(node, Aggref))
+	{
+		Aggref	   *agg = (Aggref *) node;
+
+		/* we should not consider the direct arguments, if any */
+		finalize_primnode((Node *) agg->args, context);
+		return false;			/* there can't be any Aggrefs below here */
+	}
+	return expression_tree_walker(node, finalize_agg_primnode,
+								  (void *) context);
+}
+
+/*
  * SS_make_initplan_from_plan - given a plan tree, make it an InitPlan
  *
  * The plan is expected to return a scalar value of the given type/collation.