Allow parallel workers to execute subplans.

This doesn't do anything to make Param nodes anything other than
parallel-restricted, so this only helps with uncorrelated subplans,
and it's not necessarily very cheap because each worker will run the
subplan separately (just as a Hash Join will build a separate copy of
the hash table in each participating process), but it's a first step
toward supporting cases that are more likely to help in practice, and
is occasionally useful on its own.

Amit Kapila, reviewed and tested by Rafia Sabih, Dilip Kumar, and
me.

Discussion: http://postgr.es/m/CAA4eK1+e8Z45D2n+rnDMDYsVEb5iW7jqaCH_tvPMYau=1Rru9w@mail.gmail.com

src/backend/executor/execParallel.c

@@ -156,7 +156,7 @@ ExecSerializePlan(Plan *plan, EState *estate)
 	pstmt->planTree = plan;
 	pstmt->rtable = estate->es_range_table;
 	pstmt->resultRelations = NIL;
-	pstmt->subplans = NIL;
+	pstmt->subplans = estate->es_plannedstmt->subplans;
 	pstmt->rewindPlanIDs = NULL;
 	pstmt->rowMarks = NIL;
 	pstmt->relationOids = NIL;

src/backend/nodes/copyfuncs.c

@@ -1495,6 +1495,7 @@ _copySubPlan(const SubPlan *from)
 	COPY_SCALAR_FIELD(firstColCollation);
 	COPY_SCALAR_FIELD(useHashTable);
 	COPY_SCALAR_FIELD(unknownEqFalse);
+	COPY_SCALAR_FIELD(parallel_safe);
 	COPY_NODE_FIELD(setParam);
 	COPY_NODE_FIELD(parParam);
 	COPY_NODE_FIELD(args);

src/backend/nodes/equalfuncs.c

@@ -423,6 +423,7 @@ _equalSubPlan(const SubPlan *a, const SubPlan *b)
 	COMPARE_SCALAR_FIELD(firstColCollation);
 	COMPARE_SCALAR_FIELD(useHashTable);
 	COMPARE_SCALAR_FIELD(unknownEqFalse);
+	COMPARE_SCALAR_FIELD(parallel_safe);
 	COMPARE_NODE_FIELD(setParam);
 	COMPARE_NODE_FIELD(parParam);
 	COMPARE_NODE_FIELD(args);

src/backend/nodes/outfuncs.c

@@ -1226,6 +1226,7 @@ _outSubPlan(StringInfo str, const SubPlan *node)
 	WRITE_OID_FIELD(firstColCollation);
 	WRITE_BOOL_FIELD(useHashTable);
 	WRITE_BOOL_FIELD(unknownEqFalse);
+	WRITE_BOOL_FIELD(parallel_safe);
 	WRITE_NODE_FIELD(setParam);
 	WRITE_NODE_FIELD(parParam);
 	WRITE_NODE_FIELD(args);

src/backend/nodes/readfuncs.c

@@ -2233,6 +2233,7 @@ _readSubPlan(void)
 	READ_OID_FIELD(firstColCollation);
 	READ_BOOL_FIELD(useHashTable);
 	READ_BOOL_FIELD(unknownEqFalse);
+	READ_BOOL_FIELD(parallel_safe);
 	READ_NODE_FIELD(setParam);
 	READ_NODE_FIELD(parParam);
 	READ_NODE_FIELD(args);

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;

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.

src/include/nodes/primnodes.h

@@ -677,6 +677,7 @@ typedef struct SubPlan
 	bool		unknownEqFalse; /* TRUE if it's okay to return FALSE when the
 								 * spec result is UNKNOWN; this allows much
 								 * simpler handling of null values */
+	bool		parallel_safe;	/* OK to use as part of parallel plan? */
 	/* Information for passing params into and out of the subselect: */
 	/* setParam and parParam are lists of integers (param IDs) */
 	List	   *setParam;		/* initplan subqueries have to set these

src/test/regress/expected/select_parallel.out

@@ -99,6 +99,32 @@ explain (costs off)
    ->  Index Only Scan using tenk1_unique1 on tenk1
 (3 rows)
 
+-- test parallel plans for queries containing un-correlated subplans.
+alter table tenk2 set (parallel_workers = 0);
+explain (costs off)
+	select count(*) from tenk1 where (two, four) not in
+	(select hundred, thousand from tenk2 where thousand > 100);
+                      QUERY PLAN                      
+------------------------------------------------------
+ Finalize Aggregate
+   ->  Gather
+         Workers Planned: 4
+         ->  Partial Aggregate
+               ->  Parallel Seq Scan on tenk1
+                     Filter: (NOT (hashed SubPlan 1))
+                     SubPlan 1
+                       ->  Seq Scan on tenk2
+                             Filter: (thousand > 100)
+(9 rows)
+
+select count(*) from tenk1 where (two, four) not in
+	(select hundred, thousand from tenk2 where thousand > 100);
+ count 
+-------
+ 10000
+(1 row)
+
+alter table tenk2 reset (parallel_workers);
 set force_parallel_mode=1;
 explain (costs off)
   select stringu1::int2 from tenk1 where unique1 = 1;

src/test/regress/sql/select_parallel.sql

@@ -39,6 +39,15 @@ explain (costs off)
 	select  sum(parallel_restricted(unique1)) from tenk1
 	group by(parallel_restricted(unique1));
 
+-- test parallel plans for queries containing un-correlated subplans.
+alter table tenk2 set (parallel_workers = 0);
+explain (costs off)
+	select count(*) from tenk1 where (two, four) not in
+	(select hundred, thousand from tenk2 where thousand > 100);
+select count(*) from tenk1 where (two, four) not in
+	(select hundred, thousand from tenk2 where thousand > 100);
+alter table tenk2 reset (parallel_workers);
+
 set force_parallel_mode=1;
 
 explain (costs off)