2 files changed, 20 insertions, 13 deletions

diff --git a/src/backend/optimizer/plan/subselect.c b/src/backend/optimizer/plan/subselect.c
index 9fc748973e..7954c445dd 100644
--- a/src/backend/optimizer/plan/subselect.c
+++ b/src/backend/optimizer/plan/subselect.c
@@ -58,7 +58,7 @@ static Node *build_subplan(PlannerInfo *root, Plan *plan, PlannerInfo *subroot,
 			  List *plan_params,
 			  SubLinkType subLinkType, int subLinkId,
 			  Node *testexpr, bool adjust_testexpr,
-			  bool unknownEqFalse);
+			  bool unknownEqFalse, bool parallel_safe);
 static List *generate_subquery_params(PlannerInfo *root, List *tlist,
 						 List **paramIds);
 static List *generate_subquery_vars(PlannerInfo *root, List *tlist,
@@ -551,7 +551,8 @@ make_subplan(PlannerInfo *root, Query *orig_subquery,
 	/* And convert to SubPlan or InitPlan format. */
 	result = build_subplan(root, plan, subroot, plan_params,
 						   subLinkType, subLinkId,
-						   testexpr, true, isTopQual);
+						   testexpr, true, isTopQual,
+						   best_path->parallel_safe);
 
 	/*
 	 * If it's a correlated EXISTS with an unimportant targetlist, we might be
@@ -604,7 +605,8 @@ make_subplan(PlannerInfo *root, Query *orig_subquery,
 													 plan_params,
 													 ANY_SUBLINK, 0,
 													 newtestexpr,
-													 false, true);
+													 false, true,
+												   best_path->parallel_safe);
 				/* Check we got what we expected */
 				Assert(IsA(hashplan, SubPlan));
 				Assert(hashplan->parParam == NIL);
@@ -634,7 +636,7 @@ build_subplan(PlannerInfo *root, Plan *plan, PlannerInfo *subroot,
 			  List *plan_params,
 			  SubLinkType subLinkType, int subLinkId,
 			  Node *testexpr, bool adjust_testexpr,
-			  bool unknownEqFalse)
+			  bool unknownEqFalse, bool parallel_safe)
 {
 	Node	   *result;
 	SubPlan    *splan;
@@ -653,6 +655,7 @@ build_subplan(PlannerInfo *root, Plan *plan, PlannerInfo *subroot,
 					   &splan->firstColCollation);
 	splan->useHashTable = false;
 	splan->unknownEqFalse = unknownEqFalse;
+	splan->parallel_safe = parallel_safe;
 	splan->setParam = NIL;
 	splan->parParam = NIL;
 	splan->args = NIL;
@@ -1213,6 +1216,12 @@ SS_process_ctes(PlannerInfo *root)
 						   &splan->firstColCollation);
 		splan->useHashTable = false;
 		splan->unknownEqFalse = false;
+
+		/*
+		 * CTE scans are not considered for parallelism (cf
+		 * set_rel_consider_parallel).
+		 */
+		splan->parallel_safe = false;
 		splan->setParam = NIL;
 		splan->parParam = NIL;
 		splan->args = NIL;
diff --git a/src/backend/optimizer/util/clauses.c b/src/backend/optimizer/util/clauses.c
index d589dc2544..3dedee6d69 100644
--- a/src/backend/optimizer/util/clauses.c
+++ b/src/backend/optimizer/util/clauses.c
@@ -1162,21 +1162,19 @@ max_parallel_hazard_walker(Node *node, max_parallel_hazard_context *context)
 	}
 
 	/*
-	 * Since we don't have the ability to push subplans down to workers at
-	 * present, we treat subplan references as parallel-restricted.  We need
-	 * not worry about examining their contents; if they are unsafe, we would
-	 * have found that out while examining the whole tree before reduction of
-	 * sublinks to subplans.  (Really we should not see SubLink during a
-	 * max_interesting == restricted scan, but if we do, return true.)
+	 * Really we should not see SubLink during a max_interesting == restricted
+	 * scan, but if we do, return true.
 	 */
-	else if (IsA(node, SubLink) ||
-			 IsA(node, SubPlan) ||
-			 IsA(node, AlternativeSubPlan))
+	else if (IsA(node, SubLink))
 	{
 		if (max_parallel_hazard_test(PROPARALLEL_RESTRICTED, context))
 			return true;
 	}
 
+	/* We can push the subplans only if they are parallel-safe. */
+	else if (IsA(node, SubPlan))
+		return !((SubPlan *) node)->parallel_safe;
+
 	/*
 	 * We can't pass Params to workers at the moment either, so they are also
 	 * parallel-restricted.