v1-0002-Parallel-query-support-for-basic-correlated-subqu.patch

From e3df76aa8823ff05d311fd3aa4505a7f7d58b015 Mon Sep 17 00:00:00 2001
From: jcoleman <jtc331@gmail.com>
Date: Fri, 27 Nov 2020 18:44:30 -0500
Subject: [PATCH v1 2/3] Parallel query support for basic correlated subqueries

Not all Params are inherently parallel-unsafe, but we can't know whether
they're parallel-safe up-front: we need contextual information for a
given path shape. Here we delay the final determination of whether or
not a Param is parallel-safe by initially verifying that it is minimally
parallel-safe for things that are inherent (e.g., no parallel-unsafe
functions or relations involved) and later re-checking that a given
usage is contextually safe (e.g., the Param is for correlation that can
happen entirely within a parallel worker (as opposed to needing to pass
values between workers).
---
 doc/src/sgml/parallel.sgml                    |   3 +-
 src/backend/nodes/copyfuncs.c                 |   2 +
 src/backend/nodes/equalfuncs.c                |   1 +
 src/backend/nodes/outfuncs.c                  |   3 +
 src/backend/nodes/readfuncs.c                 |   2 +
 src/backend/optimizer/path/allpaths.c         |  61 +++++--
 src/backend/optimizer/path/equivclass.c       |   4 +-
 src/backend/optimizer/path/indxpath.c         |  20 ++-
 src/backend/optimizer/path/joinpath.c         |  21 ++-
 src/backend/optimizer/plan/createplan.c       |   2 +
 src/backend/optimizer/plan/planmain.c         |   2 +-
 src/backend/optimizer/plan/planner.c          | 155 +++++++++++++++---
 src/backend/optimizer/plan/subselect.c        |  21 ++-
 src/backend/optimizer/prep/prepunion.c        |   1 +
 src/backend/optimizer/util/clauses.c          | 113 ++++++++++---
 src/backend/optimizer/util/pathnode.c         |  36 +++-
 src/backend/optimizer/util/relnode.c          |  30 +++-
 src/backend/utils/misc/guc.c                  |  11 ++
 src/include/nodes/pathnodes.h                 |   5 +-
 src/include/nodes/plannodes.h                 |   1 +
 src/include/nodes/primnodes.h                 |   1 +
 src/include/optimizer/clauses.h               |   4 +-
 .../regress/expected/incremental_sort.out     |  28 ++--
 src/test/regress/expected/select_parallel.out | 137 ++++++++++++++++
 src/test/regress/expected/sysviews.out        |   3 +-
 src/test/regress/sql/select_parallel.sql      |  37 +++++
 26 files changed, 604 insertions(+), 100 deletions(-)

diff --git a/doc/src/sgml/parallel.sgml b/doc/src/sgml/parallel.sgml
index 479e24a1dc..25a689f690 100644
--- a/doc/src/sgml/parallel.sgml
+++ b/doc/src/sgml/parallel.sgml
@@ -517,7 +517,8 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
 
     <listitem>
       <para>
-        Plan nodes which reference a correlated <literal>SubPlan</literal>.
+        Plan nodes which reference a correlated <literal>SubPlan</literal> where
+        the result is shared between workers.
       </para>
     </listitem>
   </itemizedlist>
diff --git a/src/backend/nodes/copyfuncs.c b/src/backend/nodes/copyfuncs.c
index 632cc31a04..240895ac83 100644
--- a/src/backend/nodes/copyfuncs.c
+++ b/src/backend/nodes/copyfuncs.c
@@ -121,6 +121,7 @@ CopyPlanFields(const Plan *from, Plan *newnode)
 	COPY_SCALAR_FIELD(parallel_aware);
 	COPY_SCALAR_FIELD(parallel_safe);
 	COPY_SCALAR_FIELD(async_capable);
+	COPY_SCALAR_FIELD(parallel_safe_ignoring_params);
 	COPY_SCALAR_FIELD(plan_node_id);
 	COPY_NODE_FIELD(targetlist);
 	COPY_NODE_FIELD(qual);
@@ -1777,6 +1778,7 @@ _copySubPlan(const SubPlan *from)
 	COPY_SCALAR_FIELD(useHashTable);
 	COPY_SCALAR_FIELD(unknownEqFalse);
 	COPY_SCALAR_FIELD(parallel_safe);
+	COPY_SCALAR_FIELD(parallel_safe_ignoring_params);
 	COPY_NODE_FIELD(setParam);
 	COPY_NODE_FIELD(parParam);
 	COPY_NODE_FIELD(args);
diff --git a/src/backend/nodes/equalfuncs.c b/src/backend/nodes/equalfuncs.c
index a410a29a17..011befabf2 100644
--- a/src/backend/nodes/equalfuncs.c
+++ b/src/backend/nodes/equalfuncs.c
@@ -459,6 +459,7 @@ _equalSubPlan(const SubPlan *a, const SubPlan *b)
 	COMPARE_SCALAR_FIELD(useHashTable);
 	COMPARE_SCALAR_FIELD(unknownEqFalse);
 	COMPARE_SCALAR_FIELD(parallel_safe);
+	COMPARE_SCALAR_FIELD(parallel_safe_ignoring_params);
 	COMPARE_NODE_FIELD(setParam);
 	COMPARE_NODE_FIELD(parParam);
 	COMPARE_NODE_FIELD(args);
diff --git a/src/backend/nodes/outfuncs.c b/src/backend/nodes/outfuncs.c
index c723f6d635..84156d6465 100644
--- a/src/backend/nodes/outfuncs.c
+++ b/src/backend/nodes/outfuncs.c
@@ -334,6 +334,7 @@ _outPlanInfo(StringInfo str, const Plan *node)
 	WRITE_BOOL_FIELD(parallel_aware);
 	WRITE_BOOL_FIELD(parallel_safe);
 	WRITE_BOOL_FIELD(async_capable);
+	WRITE_BOOL_FIELD(parallel_safe_ignoring_params);
 	WRITE_INT_FIELD(plan_node_id);
 	WRITE_NODE_FIELD(targetlist);
 	WRITE_NODE_FIELD(qual);
@@ -1372,6 +1373,7 @@ _outSubPlan(StringInfo str, const SubPlan *node)
 	WRITE_BOOL_FIELD(useHashTable);
 	WRITE_BOOL_FIELD(unknownEqFalse);
 	WRITE_BOOL_FIELD(parallel_safe);
+	WRITE_BOOL_FIELD(parallel_safe_ignoring_params);
 	WRITE_NODE_FIELD(setParam);
 	WRITE_NODE_FIELD(parParam);
 	WRITE_NODE_FIELD(args);
@@ -1770,6 +1772,7 @@ _outPathInfo(StringInfo str, const Path *node)
 		outBitmapset(str, NULL);
 	WRITE_BOOL_FIELD(parallel_aware);
 	WRITE_BOOL_FIELD(parallel_safe);
+	WRITE_BOOL_FIELD(parallel_safe_ignoring_params);
 	WRITE_INT_FIELD(parallel_workers);
 	WRITE_FLOAT_FIELD(rows, "%.0f");
 	WRITE_FLOAT_FIELD(startup_cost, "%.2f");
diff --git a/src/backend/nodes/readfuncs.c b/src/backend/nodes/readfuncs.c
index 3746668f52..2b22978616 100644
--- a/src/backend/nodes/readfuncs.c
+++ b/src/backend/nodes/readfuncs.c
@@ -1620,6 +1620,7 @@ ReadCommonPlan(Plan *local_node)
 	READ_BOOL_FIELD(parallel_aware);
 	READ_BOOL_FIELD(parallel_safe);
 	READ_BOOL_FIELD(async_capable);
+	READ_BOOL_FIELD(parallel_safe_ignoring_params);
 	READ_INT_FIELD(plan_node_id);
 	READ_NODE_FIELD(targetlist);
 	READ_NODE_FIELD(qual);
@@ -2613,6 +2614,7 @@ _readSubPlan(void)
 	READ_BOOL_FIELD(useHashTable);
 	READ_BOOL_FIELD(unknownEqFalse);
 	READ_BOOL_FIELD(parallel_safe);
+	READ_BOOL_FIELD(parallel_safe_ignoring_params);
 	READ_NODE_FIELD(setParam);
 	READ_NODE_FIELD(parParam);
 	READ_NODE_FIELD(args);
diff --git a/src/backend/optimizer/path/allpaths.c b/src/backend/optimizer/path/allpaths.c
index 45e37590d8..ae744da135 100644
--- a/src/backend/optimizer/path/allpaths.c
+++ b/src/backend/optimizer/path/allpaths.c
@@ -556,7 +556,8 @@ set_rel_pathlist(PlannerInfo *root, RelOptInfo *rel,
 	 * (see grouping_planner).
 	 */
 	if (rel->reloptkind == RELOPT_BASEREL &&
-		bms_membership(root->all_baserels) != BMS_SINGLETON)
+		bms_membership(root->all_baserels) != BMS_SINGLETON
+		&& (rel->subplan_params == NIL || rte->rtekind != RTE_SUBQUERY))
 		generate_useful_gather_paths(root, rel, false);
 
 	/* Now find the cheapest of the paths for this rel */
@@ -592,6 +593,9 @@ static void
 set_rel_consider_parallel(PlannerInfo *root, RelOptInfo *rel,
 						  RangeTblEntry *rte)
 {
+	bool parallel_safe;
+	bool parallel_safe_except_in_params;
+
 	/*
 	 * The flag has previously been initialized to false, so we can just
 	 * return if it becomes clear that we can't safely set it.
@@ -632,7 +636,7 @@ set_rel_consider_parallel(PlannerInfo *root, RelOptInfo *rel,
 
 				if (proparallel != PROPARALLEL_SAFE)
 					return;
-				if (!is_parallel_safe(root, (Node *) rte->tablesample->args))
+				if (!is_parallel_safe(root, (Node *) rte->tablesample->args, NULL))
 					return;
 			}
 
@@ -700,7 +704,7 @@ set_rel_consider_parallel(PlannerInfo *root, RelOptInfo *rel,
 
 		case RTE_FUNCTION:
 			/* Check for parallel-restricted functions. */
-			if (!is_parallel_safe(root, (Node *) rte->functions))
+			if (!is_parallel_safe(root, (Node *) rte->functions, NULL))
 				return;
 			break;
 
@@ -710,7 +714,7 @@ set_rel_consider_parallel(PlannerInfo *root, RelOptInfo *rel,
 
 		case RTE_VALUES:
 			/* Check for parallel-restricted functions. */
-			if (!is_parallel_safe(root, (Node *) rte->values_lists))
+			if (!is_parallel_safe(root, (Node *) rte->values_lists, NULL))
 				return;
 			break;
 
@@ -747,18 +751,28 @@ set_rel_consider_parallel(PlannerInfo *root, RelOptInfo *rel,
 	 * outer join clauses work correctly.  It would likely break equivalence
 	 * classes, too.
 	 */
-	if (!is_parallel_safe(root, (Node *) rel->baserestrictinfo))
-		return;
+	parallel_safe = is_parallel_safe(root, (Node *) rel->baserestrictinfo,
+			&parallel_safe_except_in_params);
 
 	/*
 	 * Likewise, if the relation's outputs are not parallel-safe, give up.
 	 * (Usually, they're just Vars, but sometimes they're not.)
 	 */
-	if (!is_parallel_safe(root, (Node *) rel->reltarget->exprs))
-		return;
+	if (parallel_safe || parallel_safe_except_in_params)
+	{
+		bool target_parallel_safe;
+		bool target_parallel_safe_ignoring_params = false;
+
+		target_parallel_safe = is_parallel_safe(root,
+				(Node *) rel->reltarget->exprs,
+				&target_parallel_safe_ignoring_params);
+		parallel_safe = parallel_safe && target_parallel_safe;
+		parallel_safe_except_in_params = parallel_safe_except_in_params
+			&& target_parallel_safe_ignoring_params;
+	}
 
-	/* We have a winner. */
-	rel->consider_parallel = true;
+	rel->consider_parallel = parallel_safe;
+	rel->consider_parallel_rechecking_params = parallel_safe_except_in_params;
 }
 
 /*
@@ -2277,9 +2291,21 @@ set_subquery_pathlist(PlannerInfo *root, RelOptInfo *rel,
 										  pathkeys, required_outer));
 	}
 
+	/*
+	 * XXX: As far as I can tell, the only time partial paths exist here
+	 * is when we're going to execute multiple partial parths in parallel
+	 * under a gather node (instead of executing paths serially under
+	 * an append node). That means that the subquery scan path here
+	 * is self-contained at this point -- so by definition it can't be
+	 * reliant on lateral relids, which means we'll never have to consider
+	 * rechecking params here.
+	 */
+	Assert(!(rel->consider_parallel_rechecking_params && rel->partial_pathlist && !rel->consider_parallel));
+
 	/* If outer rel allows parallelism, do same for partial paths. */
 	if (rel->consider_parallel && bms_is_empty(required_outer))
 	{
+
 		/* If consider_parallel is false, there should be no partial paths. */
 		Assert(sub_final_rel->consider_parallel ||
 			   sub_final_rel->partial_pathlist == NIL);
@@ -2633,7 +2659,7 @@ generate_gather_paths(PlannerInfo *root, RelOptInfo *rel, bool override_rows)
 		cheapest_partial_path->rows * cheapest_partial_path->parallel_workers;
 	simple_gather_path = (Path *)
 		create_gather_path(root, rel, cheapest_partial_path, rel->reltarget,
-						   NULL, rowsp);
+						   rel->lateral_relids, rowsp);
 	add_path(rel, simple_gather_path);
 
 	/*
@@ -2650,7 +2676,7 @@ generate_gather_paths(PlannerInfo *root, RelOptInfo *rel, bool override_rows)
 
 		rows = subpath->rows * subpath->parallel_workers;
 		path = create_gather_merge_path(root, rel, subpath, rel->reltarget,
-										subpath->pathkeys, NULL, rowsp);
+										subpath->pathkeys, rel->lateral_relids, rowsp);
 		add_path(rel, &path->path);
 	}
 }
@@ -2752,11 +2778,15 @@ generate_useful_gather_paths(PlannerInfo *root, RelOptInfo *rel, bool override_r
 	double	   *rowsp = NULL;
 	List	   *useful_pathkeys_list = NIL;
 	Path	   *cheapest_partial_path = NULL;
+	Relids		required_outer = rel->lateral_relids;
 
 	/* If there are no partial paths, there's nothing to do here. */
 	if (rel->partial_pathlist == NIL)
 		return;
 
+	if (!bms_is_subset(required_outer, rel->relids))
+		return;
+
 	/* Should we override the rel's rowcount estimate? */
 	if (override_rows)
 		rowsp = &rows;
@@ -2828,7 +2858,7 @@ generate_useful_gather_paths(PlannerInfo *root, RelOptInfo *rel, bool override_r
 												tmp,
 												rel->reltarget,
 												tmp->pathkeys,
-												NULL,
+												required_outer,
 												rowsp);
 
 				add_path(rel, &path->path);
@@ -2862,7 +2892,7 @@ generate_useful_gather_paths(PlannerInfo *root, RelOptInfo *rel, bool override_r
 												tmp,
 												rel->reltarget,
 												tmp->pathkeys,
-												NULL,
+												required_outer,
 												rowsp);
 
 				add_path(rel, &path->path);
@@ -3041,7 +3071,8 @@ standard_join_search(PlannerInfo *root, int levels_needed, List *initial_rels)
 			/*
 			 * Except for the topmost scan/join rel, consider gathering
 			 * partial paths.  We'll do the same for the topmost scan/join rel
-			 * once we know the final targetlist (see grouping_planner).
+			 * once we know the final targetlist (see
+			 * apply_scanjoin_target_to_paths).
 			 */
 			if (lev < levels_needed)
 				generate_useful_gather_paths(root, rel, false);
diff --git a/src/backend/optimizer/path/equivclass.c b/src/backend/optimizer/path/equivclass.c
index 6f1abbe47d..4c176d2d49 100644
--- a/src/backend/optimizer/path/equivclass.c
+++ b/src/backend/optimizer/path/equivclass.c
@@ -897,7 +897,7 @@ find_computable_ec_member(PlannerInfo *root,
 		 * check this last because it's a rather expensive test.
 		 */
 		if (require_parallel_safe &&
-			!is_parallel_safe(root, (Node *) em->em_expr))
+			!is_parallel_safe(root, (Node *) em->em_expr, NULL))
 			continue;
 
 		return em;				/* found usable expression */
@@ -1012,7 +1012,7 @@ relation_can_be_sorted_early(PlannerInfo *root, RelOptInfo *rel,
 		 * check this last because it's a rather expensive test.
 		 */
 		if (require_parallel_safe &&
-			!is_parallel_safe(root, (Node *) em->em_expr))
+			!is_parallel_safe(root, (Node *) em->em_expr, NULL))
 			continue;
 
 		return true;
diff --git a/src/backend/optimizer/path/indxpath.c b/src/backend/optimizer/path/indxpath.c
index 0e4e00eaf0..262c129ff5 100644
--- a/src/backend/optimizer/path/indxpath.c
+++ b/src/backend/optimizer/path/indxpath.c
@@ -1047,10 +1047,21 @@ build_index_paths(PlannerInfo *root, RelOptInfo *rel,
 		/*
 		 * If appropriate, consider parallel index scan.  We don't allow
 		 * parallel index scan for bitmap index scans.
+		 *
+		 * XXX: Checking rel->consider_parallel_rechecking_params here resulted
+		 * in some odd behavior on:
+		 * select (select t.unique1 from tenk1 where tenk1.unique1 = t.unique1) from tenk1 t;
+		 * where the total cost on the chosen plan went *up* considering
+		 * the extra path.
+		 *
+		 * Current working theory is that this method is about base relation
+		 * scans, and we only want parameterized paths to be parallelized as
+		 * companions to existing parallel plans and so don't really care to
+		 * consider a separate parallel index scan here.
 		 */
 		if (index->amcanparallel &&
-			rel->consider_parallel && outer_relids == NULL &&
-			scantype != ST_BITMAPSCAN)
+				rel->consider_parallel && outer_relids == NULL &&
+				scantype != ST_BITMAPSCAN)
 		{
 			ipath = create_index_path(root, index,
 									  index_clauses,
@@ -1100,9 +1111,10 @@ build_index_paths(PlannerInfo *root, RelOptInfo *rel,
 			result = lappend(result, ipath);
 
 			/* If appropriate, consider parallel index scan */
+			/* XXX: As above here for rel->consider_parallel_rechecking_params? */
 			if (index->amcanparallel &&
-				rel->consider_parallel && outer_relids == NULL &&
-				scantype != ST_BITMAPSCAN)
+					rel->consider_parallel && outer_relids == NULL &&
+					scantype != ST_BITMAPSCAN)
 			{
 				ipath = create_index_path(root, index,
 										  index_clauses,
diff --git a/src/backend/optimizer/path/joinpath.c b/src/backend/optimizer/path/joinpath.c
index 4c30c65564..c6af979714 100644
--- a/src/backend/optimizer/path/joinpath.c
+++ b/src/backend/optimizer/path/joinpath.c
@@ -690,6 +690,7 @@ try_partial_nestloop_path(PlannerInfo *root,
 		else
 			outerrelids = outerrel->relids;
 
+		/* TODO: recheck parallel safety here? */
 		if (!bms_is_subset(inner_paramrels, outerrelids))
 			return;
 	}
@@ -1714,16 +1715,24 @@ match_unsorted_outer(PlannerInfo *root,
 	 * partial path and the joinrel is parallel-safe.  However, we can't
 	 * handle JOIN_UNIQUE_OUTER, because the outer path will be partial, and
 	 * therefore we won't be able to properly guarantee uniqueness.  Nor can
-	 * we handle joins needing lateral rels, since partial paths must not be
-	 * parameterized. Similarly, we can't handle JOIN_FULL and JOIN_RIGHT,
-	 * because they can produce false null extended rows.
+	 * we handle JOIN_FULL and JOIN_RIGHT, because they can produce false null
+	 * extended rows.
+	 *
+	 * Partial paths may only have parameters in limited cases
+	 * where the parameterization is fully satisfied without sharing state
+	 * between workers, so we only allow lateral rels on inputs to the join
+	 * if the resulting join contains no lateral rels, the inner rel's laterals
+	 * are fully satisfied by the outer rel, and the outer rel doesn't depend
+	 * on the inner rel to produce any laterals.
 	 */
 	if (joinrel->consider_parallel &&
 		save_jointype != JOIN_UNIQUE_OUTER &&
 		save_jointype != JOIN_FULL &&
 		save_jointype != JOIN_RIGHT &&
 		outerrel->partial_pathlist != NIL &&
-		bms_is_empty(joinrel->lateral_relids))
+		bms_is_empty(joinrel->lateral_relids) &&
+		bms_is_subset(innerrel->lateral_relids, outerrel->relids) &&
+		(bms_is_empty(outerrel->lateral_relids) || !bms_is_subset(outerrel->lateral_relids, innerrel->relids)))
 	{
 		if (nestjoinOK)
 			consider_parallel_nestloop(root, joinrel, outerrel, innerrel,
@@ -1838,7 +1847,9 @@ consider_parallel_nestloop(PlannerInfo *root,
 			Path	   *rcpath;
 
 			/* Can't join to an inner path that is not parallel-safe */
-			if (!innerpath->parallel_safe)
+			/* TODO: recheck parallel safety of params here? */
+			if (!innerpath->parallel_safe &&
+					!(innerpath->parallel_safe_ignoring_params))
 				continue;
 
 			/*
diff --git a/src/backend/optimizer/plan/createplan.c b/src/backend/optimizer/plan/createplan.c
index a9aff24831..817ce04596 100644
--- a/src/backend/optimizer/plan/createplan.c
+++ b/src/backend/optimizer/plan/createplan.c
@@ -5274,6 +5274,7 @@ copy_generic_path_info(Plan *dest, Path *src)
 	dest->plan_width = src->pathtarget->width;
 	dest->parallel_aware = src->parallel_aware;
 	dest->parallel_safe = src->parallel_safe;
+	dest->parallel_safe_ignoring_params = src->parallel_safe_ignoring_params;
 }
 
 /*
@@ -5291,6 +5292,7 @@ copy_plan_costsize(Plan *dest, Plan *src)
 	dest->parallel_aware = false;
 	/* Assume the inserted node is parallel-safe, if child plan is. */
 	dest->parallel_safe = src->parallel_safe;
+	dest->parallel_safe_ignoring_params = src->parallel_safe_ignoring_params;
 }
 
 /*
diff --git a/src/backend/optimizer/plan/planmain.c b/src/backend/optimizer/plan/planmain.c
index 273ac0acf7..bdbce2b87d 100644
--- a/src/backend/optimizer/plan/planmain.c
+++ b/src/backend/optimizer/plan/planmain.c
@@ -119,7 +119,7 @@ query_planner(PlannerInfo *root,
 				if (root->glob->parallelModeOK &&
 					force_parallel_mode != FORCE_PARALLEL_OFF)
 					final_rel->consider_parallel =
-						is_parallel_safe(root, parse->jointree->quals);
+						is_parallel_safe(root, parse->jointree->quals, NULL);
 
 				/*
 				 * The only path for it is a trivial Result path.  We cheat a
diff --git a/src/backend/optimizer/plan/planner.c b/src/backend/optimizer/plan/planner.c
index 1868c4eff4..28b8f7d8b5 100644
--- a/src/backend/optimizer/plan/planner.c
+++ b/src/backend/optimizer/plan/planner.c
@@ -150,6 +150,7 @@ static RelOptInfo *create_grouping_paths(PlannerInfo *root,
 										 RelOptInfo *input_rel,
 										 PathTarget *target,
 										 bool target_parallel_safe,
+										 bool target_parallel_safe_ignoring_params,
 										 grouping_sets_data *gd);
 static bool is_degenerate_grouping(PlannerInfo *root);
 static void create_degenerate_grouping_paths(PlannerInfo *root,
@@ -157,6 +158,7 @@ static void create_degenerate_grouping_paths(PlannerInfo *root,
 											 RelOptInfo *grouped_rel);
 static RelOptInfo *make_grouping_rel(PlannerInfo *root, RelOptInfo *input_rel,
 									 PathTarget *target, bool target_parallel_safe,
+									 bool target_parallel_safe_ignoring_params,
 									 Node *havingQual);
 static void create_ordinary_grouping_paths(PlannerInfo *root,
 										   RelOptInfo *input_rel,
@@ -231,6 +233,7 @@ static void apply_scanjoin_target_to_paths(PlannerInfo *root,
 										   List *scanjoin_targets,
 										   List *scanjoin_targets_contain_srfs,
 										   bool scanjoin_target_parallel_safe,
+										   bool scanjoin_target_parallel_safe_ignoring_params,
 										   bool tlist_same_exprs);
 static void create_partitionwise_grouping_paths(PlannerInfo *root,
 												RelOptInfo *input_rel,
@@ -1235,6 +1238,7 @@ grouping_planner(PlannerInfo *root, double tuple_fraction)
 	List	   *final_targets;
 	List	   *final_targets_contain_srfs;
 	bool		final_target_parallel_safe;
+	bool		final_target_parallel_safe_ignoring_params = false;
 	RelOptInfo *current_rel;
 	RelOptInfo *final_rel;
 	FinalPathExtraData extra;
@@ -1297,7 +1301,7 @@ grouping_planner(PlannerInfo *root, double tuple_fraction)
 
 		/* And check whether it's parallel safe */
 		final_target_parallel_safe =
-			is_parallel_safe(root, (Node *) final_target->exprs);
+			is_parallel_safe(root, (Node *) final_target->exprs, &final_target_parallel_safe_ignoring_params);
 
 		/* The setop result tlist couldn't contain any SRFs */
 		Assert(!parse->hasTargetSRFs);
@@ -1331,14 +1335,17 @@ grouping_planner(PlannerInfo *root, double tuple_fraction)
 		List	   *sort_input_targets;
 		List	   *sort_input_targets_contain_srfs;
 		bool		sort_input_target_parallel_safe;
+		bool		sort_input_target_parallel_safe_ignoring_params = false;
 		PathTarget *grouping_target;
 		List	   *grouping_targets;
 		List	   *grouping_targets_contain_srfs;
 		bool		grouping_target_parallel_safe;
+		bool		grouping_target_parallel_safe_ignoring_params = false;
 		PathTarget *scanjoin_target;
 		List	   *scanjoin_targets;
 		List	   *scanjoin_targets_contain_srfs;
 		bool		scanjoin_target_parallel_safe;
+		bool		scanjoin_target_parallel_safe_ignoring_params = false;
 		bool		scanjoin_target_same_exprs;
 		bool		have_grouping;
 		WindowFuncLists *wflists = NULL;
@@ -1451,7 +1458,7 @@ grouping_planner(PlannerInfo *root, double tuple_fraction)
 		 */
 		final_target = create_pathtarget(root, root->processed_tlist);
 		final_target_parallel_safe =
-			is_parallel_safe(root, (Node *) final_target->exprs);
+			is_parallel_safe(root, (Node *) final_target->exprs, &final_target_parallel_safe_ignoring_params);
 
 		/*
 		 * If ORDER BY was given, consider whether we should use a post-sort
@@ -1464,12 +1471,13 @@ grouping_planner(PlannerInfo *root, double tuple_fraction)
 													   final_target,
 													   &have_postponed_srfs);
 			sort_input_target_parallel_safe =
-				is_parallel_safe(root, (Node *) sort_input_target->exprs);
+				is_parallel_safe(root, (Node *) sort_input_target->exprs, &sort_input_target_parallel_safe_ignoring_params);
 		}
 		else
 		{
 			sort_input_target = final_target;
 			sort_input_target_parallel_safe = final_target_parallel_safe;
+			sort_input_target_parallel_safe_ignoring_params = final_target_parallel_safe_ignoring_params;
 		}
 
 		/*
@@ -1483,12 +1491,13 @@ grouping_planner(PlannerInfo *root, double tuple_fraction)
 													   final_target,
 													   activeWindows);
 			grouping_target_parallel_safe =
-				is_parallel_safe(root, (Node *) grouping_target->exprs);
+				is_parallel_safe(root, (Node *) grouping_target->exprs, &grouping_target_parallel_safe_ignoring_params);
 		}
 		else
 		{
 			grouping_target = sort_input_target;
 			grouping_target_parallel_safe = sort_input_target_parallel_safe;
+			grouping_target_parallel_safe_ignoring_params = sort_input_target_parallel_safe_ignoring_params;
 		}
 
 		/*
@@ -1502,12 +1511,13 @@ grouping_planner(PlannerInfo *root, double tuple_fraction)
 		{
 			scanjoin_target = make_group_input_target(root, final_target);
 			scanjoin_target_parallel_safe =
-				is_parallel_safe(root, (Node *) scanjoin_target->exprs);
+				is_parallel_safe(root, (Node *) scanjoin_target->exprs, &scanjoin_target_parallel_safe_ignoring_params);
 		}
 		else
 		{
 			scanjoin_target = grouping_target;
 			scanjoin_target_parallel_safe = grouping_target_parallel_safe;
+			scanjoin_target_parallel_safe_ignoring_params = grouping_target_parallel_safe_ignoring_params;
 		}
 
 		/*
@@ -1559,6 +1569,7 @@ grouping_planner(PlannerInfo *root, double tuple_fraction)
 		apply_scanjoin_target_to_paths(root, current_rel, scanjoin_targets,
 									   scanjoin_targets_contain_srfs,
 									   scanjoin_target_parallel_safe,
+									   scanjoin_target_parallel_safe_ignoring_params,
 									   scanjoin_target_same_exprs);
 
 		/*
@@ -1585,6 +1596,7 @@ grouping_planner(PlannerInfo *root, double tuple_fraction)
 												current_rel,
 												grouping_target,
 												grouping_target_parallel_safe,
+												grouping_target_parallel_safe_ignoring_params,
 												gset_data);
 			/* Fix things up if grouping_target contains SRFs */
 			if (parse->hasTargetSRFs)
@@ -1658,10 +1670,25 @@ grouping_planner(PlannerInfo *root, double tuple_fraction)
 	 * not a SELECT, consider_parallel will be false for every relation in the
 	 * query.
 	 */
-	if (current_rel->consider_parallel &&
-		is_parallel_safe(root, parse->limitOffset) &&
-		is_parallel_safe(root, parse->limitCount))
-		final_rel->consider_parallel = true;
+	if (current_rel->consider_parallel || current_rel->consider_parallel_rechecking_params)
+	{
+		bool		limit_count_parallel_safe;
+		bool		limit_offset_parallel_safe;
+		bool		limit_count_parallel_safe_ignoring_params = false;
+		bool		limit_offset_parallel_safe_ignoring_params = false;
+
+		limit_count_parallel_safe = is_parallel_safe(root, parse->limitCount, &limit_count_parallel_safe_ignoring_params);
+		limit_offset_parallel_safe = is_parallel_safe(root, parse->limitOffset, &limit_offset_parallel_safe_ignoring_params);
+
+		if (current_rel->consider_parallel &&
+				limit_count_parallel_safe &&
+				limit_offset_parallel_safe)
+			final_rel->consider_parallel = true;
+		if (current_rel->consider_parallel_rechecking_params &&
+				limit_count_parallel_safe_ignoring_params &&
+				limit_offset_parallel_safe_ignoring_params)
+			final_rel->consider_parallel_rechecking_params = true;
+	}
 
 	/*
 	 * If the current_rel belongs to a single FDW, so does the final_rel.
@@ -1862,8 +1889,8 @@ grouping_planner(PlannerInfo *root, double tuple_fraction)
 	 * Generate partial paths for final_rel, too, if outer query levels might
 	 * be able to make use of them.
 	 */
-	if (final_rel->consider_parallel && root->query_level > 1 &&
-		!limit_needed(parse))
+	if ((final_rel->consider_parallel || final_rel->consider_parallel_rechecking_params) &&
+		root->query_level > 1 && !limit_needed(parse))
 	{
 		Assert(!parse->rowMarks && parse->commandType == CMD_SELECT);
 		foreach(lc, current_rel->partial_pathlist)
@@ -3275,6 +3302,7 @@ create_grouping_paths(PlannerInfo *root,
 					  RelOptInfo *input_rel,
 					  PathTarget *target,
 					  bool target_parallel_safe,
+					  bool target_parallel_safe_ignoring_params,
 					  grouping_sets_data *gd)
 {
 	Query	   *parse = root->parse;
@@ -3290,7 +3318,9 @@ create_grouping_paths(PlannerInfo *root,
 	 * aggregation paths.
 	 */
 	grouped_rel = make_grouping_rel(root, input_rel, target,
-									target_parallel_safe, parse->havingQual);
+									target_parallel_safe,
+									target_parallel_safe_ignoring_params,
+									parse->havingQual);
 
 	/*
 	 * Create either paths for a degenerate grouping or paths for ordinary
@@ -3351,6 +3381,7 @@ create_grouping_paths(PlannerInfo *root,
 
 		extra.flags = flags;
 		extra.target_parallel_safe = target_parallel_safe;
+		extra.target_parallel_safe_ignoring_params = target_parallel_safe_ignoring_params;
 		extra.havingQual = parse->havingQual;
 		extra.targetList = parse->targetList;
 		extra.partial_costs_set = false;
@@ -3386,7 +3417,7 @@ create_grouping_paths(PlannerInfo *root,
 static RelOptInfo *
 make_grouping_rel(PlannerInfo *root, RelOptInfo *input_rel,
 				  PathTarget *target, bool target_parallel_safe,
-				  Node *havingQual)
+				  bool target_parallel_safe_ignoring_params, Node *havingQual)
 {
 	RelOptInfo *grouped_rel;
 
@@ -3414,9 +3445,21 @@ make_grouping_rel(PlannerInfo *root, RelOptInfo *input_rel,
 	 * can't be parallel-safe, either.  Otherwise, it's parallel-safe if the
 	 * target list and HAVING quals are parallel-safe.
 	 */
-	if (input_rel->consider_parallel && target_parallel_safe &&
-		is_parallel_safe(root, (Node *) havingQual))
-		grouped_rel->consider_parallel = true;
+	if ((input_rel->consider_parallel || input_rel->consider_parallel_rechecking_params)
+			&& (target_parallel_safe || target_parallel_safe_ignoring_params))
+	{
+		bool having_qual_parallel_safe;
+		bool having_qual_parallel_safe_ignoring_params = false;
+
+		having_qual_parallel_safe = is_parallel_safe(root, (Node *) havingQual,
+				&having_qual_parallel_safe_ignoring_params);
+
+		grouped_rel->consider_parallel = input_rel->consider_parallel &&
+			having_qual_parallel_safe && target_parallel_safe;
+		grouped_rel->consider_parallel_rechecking_params =
+			input_rel->consider_parallel_rechecking_params &&
+			having_qual_parallel_safe_ignoring_params && target_parallel_safe_ignoring_params;
+	}
 
 	/*
 	 * If the input rel belongs to a single FDW, so does the grouped rel.
@@ -4043,7 +4086,7 @@ create_window_paths(PlannerInfo *root,
 	 * target list and active windows for non-parallel-safe constructs.
 	 */
 	if (input_rel->consider_parallel && output_target_parallel_safe &&
-		is_parallel_safe(root, (Node *) activeWindows))
+		is_parallel_safe(root, (Node *) activeWindows, NULL))
 		window_rel->consider_parallel = true;
 
 	/*
@@ -5602,6 +5645,7 @@ adjust_paths_for_srfs(PlannerInfo *root, RelOptInfo *rel,
 			PathTarget *thistarget = lfirst_node(PathTarget, lc1);
 			bool		contains_srfs = (bool) lfirst_int(lc2);
 
+			/* TODO: How do we know the new target is parallel safe? */
 			/* If this level doesn't contain SRFs, do regular projection */
 			if (contains_srfs)
 				newpath = (Path *) create_set_projection_path(root,
@@ -5918,8 +5962,8 @@ plan_create_index_workers(Oid tableOid, Oid indexOid)
 	 * safe.
 	 */
 	if (heap->rd_rel->relpersistence == RELPERSISTENCE_TEMP ||
-		!is_parallel_safe(root, (Node *) RelationGetIndexExpressions(index)) ||
-		!is_parallel_safe(root, (Node *) RelationGetIndexPredicate(index)))
+		!is_parallel_safe(root, (Node *) RelationGetIndexExpressions(index), NULL) ||
+		!is_parallel_safe(root, (Node *) RelationGetIndexPredicate(index), NULL))
 	{
 		parallel_workers = 0;
 		goto done;
@@ -6382,6 +6426,8 @@ create_partial_grouping_paths(PlannerInfo *root,
 											grouped_rel->relids);
 	partially_grouped_rel->consider_parallel =
 		grouped_rel->consider_parallel;
+	partially_grouped_rel->consider_parallel_rechecking_params =
+		grouped_rel->consider_parallel_rechecking_params;
 	partially_grouped_rel->reloptkind = grouped_rel->reloptkind;
 	partially_grouped_rel->serverid = grouped_rel->serverid;
 	partially_grouped_rel->userid = grouped_rel->userid;
@@ -6845,6 +6891,7 @@ apply_scanjoin_target_to_paths(PlannerInfo *root,
 							   List *scanjoin_targets,
 							   List *scanjoin_targets_contain_srfs,
 							   bool scanjoin_target_parallel_safe,
+							   bool scanjoin_target_parallel_safe_ignoring_params,
 							   bool tlist_same_exprs)
 {
 	bool		rel_is_partitioned = IS_PARTITIONED_REL(rel);
@@ -6854,6 +6901,11 @@ apply_scanjoin_target_to_paths(PlannerInfo *root,
 	/* This recurses, so be paranoid. */
 	check_stack_depth();
 
+	/*
+	 * TOOD: when/how do we want to generate gather paths if
+	 * scanjoin_target_parallel_safe_ignoring_params = true
+	 */
+
 	/*
 	 * If the rel is partitioned, we want to drop its existing paths and
 	 * generate new ones.  This function would still be correct if we kept the
@@ -6894,6 +6946,64 @@ apply_scanjoin_target_to_paths(PlannerInfo *root,
 		generate_useful_gather_paths(root, rel, false);
 
 		/* Can't use parallel query above this level. */
+
+		/*
+		 * There are cases where:
+		 * (rel->consider_parallel &&
+		 *  !scanjoin_target_parallel_safe &&
+		 *  scanjoin_target_parallel_safe_ignoring_params)
+		 * is true at this point. See longer commment below.
+		 */
+		if (!(rel->consider_parallel_rechecking_params && scanjoin_target_parallel_safe_ignoring_params))
+		{
+			/*
+			 * TODO: if we limit this to this condition, we're pushing off the
+			 * checks as to whether or not a given param usage is safe in the
+			 * context of a given path (in the context of a given rel?). That
+			 * almost certainly means we'd have to add other checks later (maybe
+			 * just on lateral/relids and not parallel safety overall), because
+			 * at the end of grouping_planner() we copy partial paths to the
+			 * final_rel, and while that path may be acceptable in some contexts
+			 * it may not be in all contexts.
+			 *
+			 * OTOH if we're only dependent on PARAM_EXEC params, and we already
+			 * know that subpath->param_info == NULL holds (and that seems like
+			 * it must since we were going to replace the path target anyway...
+			 * though the one caveat is from the original form of this function
+			 * we'd only ever actually assert that for paths not partial paths)
+			 * then if a param shows up in the target why would it not be parallel
+			 * safe.
+			 *
+			 * Adding to the mystery even with the original form of this function
+			 * we still end up with parallel paths where I'd expect this to
+			 * disallow them. For example:
+			 *
+			 * SELECT '' AS six, f1 AS "Correlated Field", f3 AS "Second Field"
+			 * FROM SUBSELECT_TBL upper
+			 * WHERE f3 IN (
+			 *   SELECT upper.f1 + f2
+			 *   FROM SUBSELECT_TBL
+			 *   WHERE f2 = CAST(f3 AS integer)
+			 * );
+			 *
+			 * ends up with the correlated query underneath parallel plan despite
+			 * its target containing a param, and therefore this function marking
+			 * the rel as consider_parallel=false and removing the partial paths.
+			 *
+			 * But the plan as a whole is parallel safe, and so the subplan is also
+			 * parallel safe, which means we can incorporate it into a full parallel
+			 * plan. In other words, this is a parallel safe, but not parallel aware
+			 * subplan (and regular, not parallel, seq scan inside that subplan).
+			 * It's not a partial path; it'a a full path that is executed as a subquery.
+			 *
+			 * Current conclusion: it's fine for subplans, which is the case we're
+			 * currently targeting anyway. And it might even be the only case that
+			 * matters at all.
+			 */
+			/* rel->consider_parallel_rechecking_params = false; */
+			/* rel->partial_pathlist = NIL; */
+		}
+		rel->consider_parallel_rechecking_params = false;
 		rel->partial_pathlist = NIL;
 		rel->consider_parallel = false;
 	}
@@ -7026,6 +7136,7 @@ apply_scanjoin_target_to_paths(PlannerInfo *root,
 										   child_scanjoin_targets,
 										   scanjoin_targets_contain_srfs,
 										   scanjoin_target_parallel_safe,
+										   scanjoin_target_parallel_safe_ignoring_params,
 										   tlist_same_exprs);
 
 			/* Save non-dummy children for Append paths. */
@@ -7043,6 +7154,11 @@ apply_scanjoin_target_to_paths(PlannerInfo *root,
 	 * avoid creating multiple Gather nodes within the same plan. We must do
 	 * this after all paths have been generated and before set_cheapest, since
 	 * one of the generated paths may turn out to be the cheapest one.
+	 *
+	 * TODO: This is the same problem as earlier in this function: when allowing
+	 * "parallel safe ignoring params" paths here we don't actually know we are
+	 * safe in any possible context just possibly safe in the context of the
+	 * right rel.
 	 */
 	if (rel->consider_parallel && !IS_OTHER_REL(rel))
 		generate_useful_gather_paths(root, rel, false);
@@ -7146,6 +7262,7 @@ create_partitionwise_grouping_paths(PlannerInfo *root,
 		child_grouped_rel = make_grouping_rel(root, child_input_rel,
 											  child_target,
 											  extra->target_parallel_safe,
+											  extra->target_parallel_safe_ignoring_params,
 											  child_extra.havingQual);
 
 		/* Create grouping paths for this child relation. */
diff --git a/src/backend/optimizer/plan/subselect.c b/src/backend/optimizer/plan/subselect.c
index 0881a208ac..fd1cfe6c73 100644
--- a/src/backend/optimizer/plan/subselect.c
+++ b/src/backend/optimizer/plan/subselect.c
@@ -342,6 +342,7 @@ build_subplan(PlannerInfo *root, Plan *plan, PlannerInfo *subroot,
 	splan->useHashTable = false;
 	splan->unknownEqFalse = unknownEqFalse;
 	splan->parallel_safe = plan->parallel_safe;
+	splan->parallel_safe_ignoring_params = plan->parallel_safe_ignoring_params;
 	splan->setParam = NIL;
 	splan->parParam = NIL;
 	splan->args = NIL;
@@ -1939,6 +1940,7 @@ process_sublinks_mutator(Node *node, process_sublinks_context *context)
 	{
 		SubLink    *sublink = (SubLink *) node;
 		Node	   *testexpr;
+		Node	   *result;
 
 		/*
 		 * First, recursively process the lefthand-side expressions, if any.
@@ -1950,12 +1952,29 @@ process_sublinks_mutator(Node *node, process_sublinks_context *context)
 		/*
 		 * Now build the SubPlan node and make the expr to return.
 		 */
-		return make_subplan(context->root,
+		result = make_subplan(context->root,
 							(Query *) sublink->subselect,
 							sublink->subLinkType,
 							sublink->subLinkId,
 							testexpr,
 							context->isTopQual);
+
+		/*
+		 * If planning determined that a subpath was parallel safe as long
+		 * as required params are provided by each individual worker then we
+		 * can mark the resulting subplan actually parallel safe since we now
+		 * know for certain how that path will be used.
+		 */
+		if (IsA(result, SubPlan) && !((SubPlan*)result)->parallel_safe
+				&& ((SubPlan*)result)->parallel_safe_ignoring_params
+				&& enable_parallel_params_recheck)
+		{
+			Plan *subplan = planner_subplan_get_plan(context->root, (SubPlan*)result);
+			((SubPlan*)result)->parallel_safe = is_parallel_safe(context->root, testexpr, NULL);
+			subplan->parallel_safe = ((SubPlan*)result)->parallel_safe;
+		}
+
+		return result;
 	}
 
 	/*
diff --git a/src/backend/optimizer/prep/prepunion.c b/src/backend/optimizer/prep/prepunion.c
index 037dfaacfd..a57bc1c2a8 100644
--- a/src/backend/optimizer/prep/prepunion.c
+++ b/src/backend/optimizer/prep/prepunion.c
@@ -409,6 +409,7 @@ recurse_set_operations(Node *setOp, PlannerInfo *root,
 				Assert(subpath->param_info == NULL);
 
 				/* avoid apply_projection_to_path, in case of multiple refs */
+				/* TODO: how to we know the target is parallel safe? */
 				path = (Path *) create_projection_path(root, subpath->parent,
 													   subpath, target);
 				lfirst(lc) = path;
diff --git a/src/backend/optimizer/util/clauses.c b/src/backend/optimizer/util/clauses.c
index d9ad4efc5e..4c188b85a4 100644
--- a/src/backend/optimizer/util/clauses.c
+++ b/src/backend/optimizer/util/clauses.c
@@ -54,6 +54,15 @@
 #include "utils/syscache.h"
 #include "utils/typcache.h"
 
+bool enable_parallel_params_recheck = true;
+
+typedef struct
+{
+	PlannerInfo *root;
+	AggSplit	aggsplit;
+	AggClauseCosts *costs;
+} get_agg_clause_costs_context;
+
 typedef struct
 {
 	ParamListInfo boundParams;
@@ -87,6 +96,8 @@ typedef struct
 {
 	char		max_hazard;		/* worst proparallel hazard found so far */
 	char		max_interesting;	/* worst proparallel hazard of interest */
+	char		max_hazard_ignoring_params;
+	bool		check_params_independently;
 	List	   *safe_param_ids; /* PARAM_EXEC Param IDs to treat as safe */
 } max_parallel_hazard_context;
 
@@ -620,19 +631,27 @@ max_parallel_hazard(Query *parse)
 	context.max_hazard = PROPARALLEL_SAFE;
 	context.max_interesting = PROPARALLEL_UNSAFE;
 	context.safe_param_ids = NIL;
+	context.check_params_independently = false;
 	(void) max_parallel_hazard_walker((Node *) parse, &context);
 	return context.max_hazard;
 }
 
 /*
  * is_parallel_safe
- *		Detect whether the given expr contains only parallel-safe functions
+ *		Detect whether the given expr contains only parallel safe funcions
+ *		XXX: It does more than functions?
+ *
+ * If provided, safe_ignoring_params will be set the result of running the same
+ * parallel safety checks with the exception that params will be allowed. This
+ * value is useful since params are not inherently parallel unsafe, but rather
+ * their usage context (whether or not the worker is able to provide the value)
+ * determines parallel safety.
  *
  * root->glob->maxParallelHazard must previously have been set to the
- * result of max_parallel_hazard() on the whole query.
+ * result of max_parallel_hazard() on the whole query
  */
 bool
-is_parallel_safe(PlannerInfo *root, Node *node)
+is_parallel_safe(PlannerInfo *root, Node *node, bool *safe_ignoring_params)
 {
 	max_parallel_hazard_context context;
 	PlannerInfo *proot;
@@ -649,8 +668,10 @@ is_parallel_safe(PlannerInfo *root, Node *node)
 		return true;
 	/* Else use max_parallel_hazard's search logic, but stop on RESTRICTED */
 	context.max_hazard = PROPARALLEL_SAFE;
+	context.max_hazard_ignoring_params = PROPARALLEL_SAFE;
 	context.max_interesting = PROPARALLEL_RESTRICTED;
 	context.safe_param_ids = NIL;
+	context.check_params_independently = safe_ignoring_params != NULL;
 
 	/*
 	 * The params that refer to the same or parent query level are considered
@@ -668,12 +689,17 @@ is_parallel_safe(PlannerInfo *root, Node *node)
 		}
 	}
 
-	return !max_parallel_hazard_walker(node, &context);
+	(void) max_parallel_hazard_walker(node, &context);
+
+	if (safe_ignoring_params)
+		*safe_ignoring_params = context.max_hazard_ignoring_params == PROPARALLEL_SAFE;
+
+	return context.max_hazard == PROPARALLEL_SAFE;
 }
 
 /* core logic for all parallel-hazard checks */
 static bool
-max_parallel_hazard_test(char proparallel, max_parallel_hazard_context *context)
+max_parallel_hazard_test(char proparallel, max_parallel_hazard_context *context, bool from_param)
 {
 	switch (proparallel)
 	{
@@ -684,12 +710,16 @@ max_parallel_hazard_test(char proparallel, max_parallel_hazard_context *context)
 			/* increase max_hazard to RESTRICTED */
 			Assert(context->max_hazard != PROPARALLEL_UNSAFE);
 			context->max_hazard = proparallel;
+			if (!from_param)
+				context->max_hazard_ignoring_params = proparallel;
 			/* done if we are not expecting any unsafe functions */
 			if (context->max_interesting == proparallel)
 				return true;
 			break;
 		case PROPARALLEL_UNSAFE:
 			context->max_hazard = proparallel;
+			if (!from_param)
+				context->max_hazard_ignoring_params = proparallel;
 			/* we're always done at the first unsafe construct */
 			return true;
 		default:
@@ -704,7 +734,41 @@ static bool
 max_parallel_hazard_checker(Oid func_id, void *context)
 {
 	return max_parallel_hazard_test(func_parallel(func_id),
-									(max_parallel_hazard_context *) context);
+									(max_parallel_hazard_context *) context, false);
+}
+
+static bool
+max_parallel_hazard_walker_can_short_circuit(max_parallel_hazard_context *context)
+{
+	if (!context->check_params_independently)
+		return true;
+
+	switch (context->max_hazard)
+	{
+		case PROPARALLEL_SAFE:
+			/* nothing to see here, move along */
+			break;
+		case PROPARALLEL_RESTRICTED:
+			if (context->max_interesting == PROPARALLEL_RESTRICTED)
+				return context->max_hazard_ignoring_params != PROPARALLEL_SAFE;
+
+			/*
+			 * We haven't even met our max interesting yet, so
+			 * we certainly can't short-circuit.
+			 */
+			break;
+		case PROPARALLEL_UNSAFE:
+			if (context->max_interesting == PROPARALLEL_RESTRICTED)
+				return context->max_hazard_ignoring_params != PROPARALLEL_SAFE;
+			else if (context->max_interesting == PROPARALLEL_UNSAFE)
+				return context->max_hazard_ignoring_params == PROPARALLEL_UNSAFE;
+
+			break;
+		default:
+			elog(ERROR, "unrecognized proparallel value \"%c\"", context->max_hazard);
+			break;
+	}
+	return false;
 }
 
 static bool
@@ -716,7 +780,7 @@ max_parallel_hazard_walker(Node *node, max_parallel_hazard_context *context)
 	/* Check for hazardous functions in node itself */
 	if (check_functions_in_node(node, max_parallel_hazard_checker,
 								context))
-		return true;
+		return max_parallel_hazard_walker_can_short_circuit(context);
 
 	/*
 	 * It should be OK to treat MinMaxExpr as parallel-safe, since btree
@@ -731,14 +795,14 @@ max_parallel_hazard_walker(Node *node, max_parallel_hazard_context *context)
 	 */
 	if (IsA(node, CoerceToDomain))
 	{
-		if (max_parallel_hazard_test(PROPARALLEL_RESTRICTED, context))
-			return true;
+		if (max_parallel_hazard_test(PROPARALLEL_RESTRICTED, context, false))
+			return max_parallel_hazard_walker_can_short_circuit(context);
 	}
 
 	else if (IsA(node, NextValueExpr))
 	{
-		if (max_parallel_hazard_test(PROPARALLEL_UNSAFE, context))
-			return true;
+		if (max_parallel_hazard_test(PROPARALLEL_UNSAFE, context, false))
+			return max_parallel_hazard_walker_can_short_circuit(context);
 	}
 
 	/*
@@ -751,8 +815,8 @@ max_parallel_hazard_walker(Node *node, max_parallel_hazard_context *context)
 	 */
 	else if (IsA(node, WindowFunc))
 	{
-		if (max_parallel_hazard_test(PROPARALLEL_RESTRICTED, context))
-			return true;
+		if (max_parallel_hazard_test(PROPARALLEL_RESTRICTED, context, false))
+			return max_parallel_hazard_walker_can_short_circuit(context);
 	}
 
 	/*
@@ -771,8 +835,8 @@ max_parallel_hazard_walker(Node *node, max_parallel_hazard_context *context)
 	 */
 	else if (IsA(node, SubLink))
 	{
-		if (max_parallel_hazard_test(PROPARALLEL_RESTRICTED, context))
-			return true;
+		if (max_parallel_hazard_test(PROPARALLEL_RESTRICTED, context, false))
+			return max_parallel_hazard_walker_can_short_circuit(context);
 	}
 
 	/*
@@ -787,18 +851,23 @@ max_parallel_hazard_walker(Node *node, max_parallel_hazard_context *context)
 		List	   *save_safe_param_ids;
 
 		if (!subplan->parallel_safe &&
-			max_parallel_hazard_test(PROPARALLEL_RESTRICTED, context))
+			(!enable_parallel_params_recheck || !subplan->parallel_safe_ignoring_params) &&
+			max_parallel_hazard_test(PROPARALLEL_RESTRICTED, context, false) &&
+			max_parallel_hazard_walker_can_short_circuit(context))
 			return true;
 		save_safe_param_ids = context->safe_param_ids;
 		context->safe_param_ids = list_concat_copy(context->safe_param_ids,
 												   subplan->paramIds);
-		if (max_parallel_hazard_walker(subplan->testexpr, context))
-			return true;		/* no need to restore safe_param_ids */
+		if (max_parallel_hazard_walker(subplan->testexpr, context) &&
+			max_parallel_hazard_walker_can_short_circuit(context))
+			/* no need to restore safe_param_ids */
+			return true;
+
 		list_free(context->safe_param_ids);
 		context->safe_param_ids = save_safe_param_ids;
 		/* we must also check args, but no special Param treatment there */
 		if (max_parallel_hazard_walker((Node *) subplan->args, context))
-			return true;
+			return max_parallel_hazard_walker_can_short_circuit(context);
 		/* don't want to recurse normally, so we're done */
 		return false;
 	}
@@ -820,8 +889,8 @@ max_parallel_hazard_walker(Node *node, max_parallel_hazard_context *context)
 		if (param->paramkind != PARAM_EXEC ||
 			!list_member_int(context->safe_param_ids, param->paramid))
 		{
-			if (max_parallel_hazard_test(PROPARALLEL_RESTRICTED, context))
-				return true;
+			if (max_parallel_hazard_test(PROPARALLEL_RESTRICTED, context, true))
+				return max_parallel_hazard_walker_can_short_circuit(context);
 		}
 		return false;			/* nothing to recurse to */
 	}
@@ -839,7 +908,7 @@ max_parallel_hazard_walker(Node *node, max_parallel_hazard_context *context)
 		if (query->rowMarks != NULL)
 		{
 			context->max_hazard = PROPARALLEL_UNSAFE;
-			return true;
+			return max_parallel_hazard_walker_can_short_circuit(context);
 		}
 
 		/* Recurse into subselects */
diff --git a/src/backend/optimizer/util/pathnode.c b/src/backend/optimizer/util/pathnode.c
index b248b038e0..93956f3828 100644
--- a/src/backend/optimizer/util/pathnode.c
+++ b/src/backend/optimizer/util/pathnode.c
@@ -756,10 +756,10 @@ add_partial_path(RelOptInfo *parent_rel, Path *new_path)
 	CHECK_FOR_INTERRUPTS();
 
 	/* Path to be added must be parallel safe. */
-	Assert(new_path->parallel_safe);
+	Assert(new_path->parallel_safe || new_path->parallel_safe_ignoring_params);
 
 	/* Relation should be OK for parallelism, too. */
-	Assert(parent_rel->consider_parallel);
+	Assert(parent_rel->consider_parallel || parent_rel->consider_parallel_rechecking_params);
 
 	/*
 	 * As in add_path, throw out any paths which are dominated by the new
@@ -938,6 +938,7 @@ create_seqscan_path(PlannerInfo *root, RelOptInfo *rel,
 													 required_outer);
 	pathnode->parallel_aware = parallel_workers > 0 ? true : false;
 	pathnode->parallel_safe = rel->consider_parallel;
+	pathnode->parallel_safe_ignoring_params = rel->consider_parallel_rechecking_params;
 	pathnode->parallel_workers = parallel_workers;
 	pathnode->pathkeys = NIL;	/* seqscan has unordered result */
 
@@ -1016,6 +1017,7 @@ create_index_path(PlannerInfo *root,
 														  required_outer);
 	pathnode->path.parallel_aware = false;
 	pathnode->path.parallel_safe = rel->consider_parallel;
+	pathnode->path.parallel_safe_ignoring_params = rel->consider_parallel_rechecking_params;
 	pathnode->path.parallel_workers = 0;
 	pathnode->path.pathkeys = pathkeys;
 
@@ -1868,7 +1870,7 @@ create_gather_merge_path(PlannerInfo *root, RelOptInfo *rel, Path *subpath,
 	Cost		input_startup_cost = 0;
 	Cost		input_total_cost = 0;
 
-	Assert(subpath->parallel_safe);
+	Assert(subpath->parallel_safe || subpath->parallel_safe_ignoring_params);
 	Assert(pathkeys);
 
 	pathnode->path.pathtype = T_GatherMerge;
@@ -1956,7 +1958,7 @@ create_gather_path(PlannerInfo *root, RelOptInfo *rel, Path *subpath,
 {
 	GatherPath *pathnode = makeNode(GatherPath);
 
-	Assert(subpath->parallel_safe);
+	Assert(subpath->parallel_safe || subpath->parallel_safe_ignoring_params);
 
 	pathnode->path.pathtype = T_Gather;
 	pathnode->path.parent = rel;
@@ -2003,6 +2005,8 @@ create_subqueryscan_path(PlannerInfo *root, RelOptInfo *rel, Path *subpath,
 	pathnode->path.parallel_aware = false;
 	pathnode->path.parallel_safe = rel->consider_parallel &&
 		subpath->parallel_safe;
+	pathnode->path.parallel_safe_ignoring_params = rel->consider_parallel_rechecking_params &&
+		subpath->parallel_safe_ignoring_params;
 	pathnode->path.parallel_workers = subpath->parallel_workers;
 	pathnode->path.pathkeys = pathkeys;
 	pathnode->subpath = subpath;
@@ -2420,6 +2424,8 @@ create_nestloop_path(PlannerInfo *root,
 	NestPath   *pathnode = makeNode(NestPath);
 	Relids		inner_req_outer = PATH_REQ_OUTER(inner_path);
 
+	/* TODO: Assert lateral relids subset safety? */
+
 	/*
 	 * If the inner path is parameterized by the outer, we must drop any
 	 * restrict_clauses that are due to be moved into the inner path.  We have
@@ -2460,6 +2466,8 @@ create_nestloop_path(PlannerInfo *root,
 	pathnode->path.parallel_aware = false;
 	pathnode->path.parallel_safe = joinrel->consider_parallel &&
 		outer_path->parallel_safe && inner_path->parallel_safe;
+	pathnode->path.parallel_safe_ignoring_params = joinrel->consider_parallel_rechecking_params &&
+		outer_path->parallel_safe_ignoring_params && inner_path->parallel_safe_ignoring_params;
 	/* This is a foolish way to estimate parallel_workers, but for now... */
 	pathnode->path.parallel_workers = outer_path->parallel_workers;
 	pathnode->path.pathkeys = pathkeys;
@@ -2633,6 +2641,8 @@ create_projection_path(PlannerInfo *root,
 {
 	ProjectionPath *pathnode = makeNode(ProjectionPath);
 	PathTarget *oldtarget = subpath->pathtarget;
+	bool target_parallel_safe;
+	bool target_parallel_safe_ignoring_params = false;
 
 	pathnode->path.pathtype = T_Result;
 	pathnode->path.parent = rel;
@@ -2640,9 +2650,12 @@ create_projection_path(PlannerInfo *root,
 	/* For now, assume we are above any joins, so no parameterization */
 	pathnode->path.param_info = NULL;
 	pathnode->path.parallel_aware = false;
+	target_parallel_safe = is_parallel_safe(root, (Node *) target->exprs,
+			&target_parallel_safe_ignoring_params);
 	pathnode->path.parallel_safe = rel->consider_parallel &&
-		subpath->parallel_safe &&
-		is_parallel_safe(root, (Node *) target->exprs);
+		subpath->parallel_safe && target_parallel_safe;
+	pathnode->path.parallel_safe_ignoring_params = rel->consider_parallel_rechecking_params &&
+		subpath->parallel_safe_ignoring_params && target_parallel_safe_ignoring_params;
 	pathnode->path.parallel_workers = subpath->parallel_workers;
 	/* Projection does not change the sort order */
 	pathnode->path.pathkeys = subpath->pathkeys;
@@ -2749,7 +2762,7 @@ apply_projection_to_path(PlannerInfo *root,
 	 * parallel-safe in the target expressions, then we can't.
 	 */
 	if ((IsA(path, GatherPath) || IsA(path, GatherMergePath)) &&
-		is_parallel_safe(root, (Node *) target->exprs))
+		is_parallel_safe(root, (Node *) target->exprs, NULL))
 	{
 		/*
 		 * We always use create_projection_path here, even if the subpath is
@@ -2783,7 +2796,7 @@ apply_projection_to_path(PlannerInfo *root,
 		}
 	}
 	else if (path->parallel_safe &&
-			 !is_parallel_safe(root, (Node *) target->exprs))
+			 !is_parallel_safe(root, (Node *) target->exprs, NULL))
 	{
 		/*
 		 * We're inserting a parallel-restricted target list into a path
@@ -2791,6 +2804,7 @@ apply_projection_to_path(PlannerInfo *root,
 		 * safe.
 		 */
 		path->parallel_safe = false;
+		path->parallel_safe_ignoring_params = false; /* TODO */
 	}
 
 	return path;
@@ -2823,7 +2837,7 @@ create_set_projection_path(PlannerInfo *root,
 	pathnode->path.parallel_aware = false;
 	pathnode->path.parallel_safe = rel->consider_parallel &&
 		subpath->parallel_safe &&
-		is_parallel_safe(root, (Node *) target->exprs);
+		is_parallel_safe(root, (Node *) target->exprs, NULL);
 	pathnode->path.parallel_workers = subpath->parallel_workers;
 	/* Projection does not change the sort order XXX? */
 	pathnode->path.pathkeys = subpath->pathkeys;
@@ -3100,6 +3114,8 @@ create_agg_path(PlannerInfo *root,
 	pathnode->path.parallel_aware = false;
 	pathnode->path.parallel_safe = rel->consider_parallel &&
 		subpath->parallel_safe;
+	pathnode->path.parallel_safe_ignoring_params = rel->consider_parallel_rechecking_params &&
+		subpath->parallel_safe_ignoring_params;
 	pathnode->path.parallel_workers = subpath->parallel_workers;
 	if (aggstrategy == AGG_SORTED)
 		pathnode->path.pathkeys = subpath->pathkeys;	/* preserves order */
@@ -3721,6 +3737,8 @@ create_limit_path(PlannerInfo *root, RelOptInfo *rel,
 	pathnode->path.parallel_aware = false;
 	pathnode->path.parallel_safe = rel->consider_parallel &&
 		subpath->parallel_safe;
+	pathnode->path.parallel_safe_ignoring_params = rel->consider_parallel_rechecking_params &&
+		subpath->parallel_safe_ignoring_params;
 	pathnode->path.parallel_workers = subpath->parallel_workers;
 	pathnode->path.rows = subpath->rows;
 	pathnode->path.startup_cost = subpath->startup_cost;
diff --git a/src/backend/optimizer/util/relnode.c b/src/backend/optimizer/util/relnode.c
index e105a4d5f1..d026db8759 100644
--- a/src/backend/optimizer/util/relnode.c
+++ b/src/backend/optimizer/util/relnode.c
@@ -213,6 +213,7 @@ build_simple_rel(PlannerInfo *root, int relid, RelOptInfo *parent)
 	rel->consider_startup = (root->tuple_fraction > 0);
 	rel->consider_param_startup = false;	/* might get changed later */
 	rel->consider_parallel = false; /* might get changed later */
+	rel->consider_parallel_rechecking_params = false; /* might get changed later */
 	rel->reltarget = create_empty_pathtarget();
 	rel->pathlist = NIL;
 	rel->ppilist = NIL;
@@ -616,6 +617,7 @@ build_join_rel(PlannerInfo *root,
 	joinrel->consider_startup = (root->tuple_fraction > 0);
 	joinrel->consider_param_startup = false;
 	joinrel->consider_parallel = false;
+	joinrel->consider_parallel_rechecking_params = false;
 	joinrel->reltarget = create_empty_pathtarget();
 	joinrel->pathlist = NIL;
 	joinrel->ppilist = NIL;
@@ -741,10 +743,27 @@ build_join_rel(PlannerInfo *root,
 	 * take; therefore, we should make the same decision here however we get
 	 * here.
 	 */
-	if (inner_rel->consider_parallel && outer_rel->consider_parallel &&
-		is_parallel_safe(root, (Node *) restrictlist) &&
-		is_parallel_safe(root, (Node *) joinrel->reltarget->exprs))
-		joinrel->consider_parallel = true;
+	if ((inner_rel->consider_parallel || inner_rel->consider_parallel_rechecking_params)
+			&& (outer_rel->consider_parallel || outer_rel->consider_parallel_rechecking_params))
+	{
+		bool restrictlist_parallel_safe;
+		bool restrictlist_parallel_safe_ignoring_params = false;
+		bool target_parallel_safe;
+		bool target_parallel_safe_ignoring_params = false;
+
+		restrictlist_parallel_safe = is_parallel_safe(root, (Node *) restrictlist, &restrictlist_parallel_safe_ignoring_params);
+		target_parallel_safe = is_parallel_safe(root, (Node *) joinrel->reltarget->exprs, &target_parallel_safe_ignoring_params);
+
+		if (inner_rel->consider_parallel && outer_rel->consider_parallel
+				&& restrictlist_parallel_safe && target_parallel_safe)
+			joinrel->consider_parallel = true;
+
+		if (inner_rel->consider_parallel_rechecking_params
+				&& outer_rel->consider_parallel_rechecking_params
+				&& restrictlist_parallel_safe_ignoring_params
+				&& target_parallel_safe_ignoring_params)
+			joinrel->consider_parallel_rechecking_params = true;
+	}
 
 	/* Add the joinrel to the PlannerInfo. */
 	add_join_rel(root, joinrel);
@@ -803,6 +822,7 @@ build_child_join_rel(PlannerInfo *root, RelOptInfo *outer_rel,
 	joinrel->consider_startup = (root->tuple_fraction > 0);
 	joinrel->consider_param_startup = false;
 	joinrel->consider_parallel = false;
+	joinrel->consider_parallel_rechecking_params = false;
 	joinrel->reltarget = create_empty_pathtarget();
 	joinrel->pathlist = NIL;
 	joinrel->ppilist = NIL;
@@ -889,6 +909,7 @@ build_child_join_rel(PlannerInfo *root, RelOptInfo *outer_rel,
 
 	/* Child joinrel is parallel safe if parent is parallel safe. */
 	joinrel->consider_parallel = parent_joinrel->consider_parallel;
+	joinrel->consider_parallel_rechecking_params = parent_joinrel->consider_parallel_rechecking_params;
 
 	/* Set estimates of the child-joinrel's size. */
 	set_joinrel_size_estimates(root, joinrel, outer_rel, inner_rel,
@@ -1233,6 +1254,7 @@ fetch_upper_rel(PlannerInfo *root, UpperRelationKind kind, Relids relids)
 	upperrel->consider_startup = (root->tuple_fraction > 0);
 	upperrel->consider_param_startup = false;
 	upperrel->consider_parallel = false;	/* might get changed later */
+	upperrel->consider_parallel_rechecking_params = false;	/* might get changed later */
 	upperrel->reltarget = create_empty_pathtarget();
 	upperrel->pathlist = NIL;
 	upperrel->cheapest_startup_path = NULL;
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index b130874bdc..1fbab55e2c 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -58,6 +58,7 @@
 #include "libpq/libpq.h"
 #include "libpq/pqformat.h"
 #include "miscadmin.h"
+#include "optimizer/clauses.h"
 #include "optimizer/cost.h"
 #include "optimizer/geqo.h"
 #include "optimizer/optimizer.h"
@@ -955,6 +956,16 @@ static const unit_conversion time_unit_conversion_table[] =
 
 static struct config_bool ConfigureNamesBool[] =
 {
+	{
+		{"enable_parallel_params_recheck", PGC_USERSET, QUERY_TUNING_METHOD,
+			gettext_noop("Enables the planner's rechecking of parallel safety in the presence of PARAM_EXEC params (for correlated subqueries)."),
+			NULL,
+			GUC_EXPLAIN
+		},
+		&enable_parallel_params_recheck,
+		true,
+		NULL, NULL, NULL
+	},
 	{
 		{"enable_seqscan", PGC_USERSET, QUERY_TUNING_METHOD,
 			gettext_noop("Enables the planner's use of sequential-scan plans."),
diff --git a/src/include/nodes/pathnodes.h b/src/include/nodes/pathnodes.h
index a65bda7e3c..2ecbb016a2 100644
--- a/src/include/nodes/pathnodes.h
+++ b/src/include/nodes/pathnodes.h
@@ -682,6 +682,7 @@ typedef struct RelOptInfo
 	bool		consider_startup;	/* keep cheap-startup-cost paths? */
 	bool		consider_param_startup; /* ditto, for parameterized paths? */
 	bool		consider_parallel;	/* consider parallel paths? */
+	bool		consider_parallel_rechecking_params;	/* consider parallel paths? */
 
 	/* default result targetlist for Paths scanning this relation */
 	struct PathTarget *reltarget;	/* list of Vars/Exprs, cost, width */
@@ -1178,6 +1179,7 @@ typedef struct Path
 
 	bool		parallel_aware; /* engage parallel-aware logic? */
 	bool		parallel_safe;	/* OK to use as part of parallel plan? */
+	bool		parallel_safe_ignoring_params;	/* OK to use as part of parallel plan if worker context provides params? */
 	int			parallel_workers;	/* desired # of workers; 0 = not parallel */
 
 	/* estimated size/costs for path (see costsize.c for more info) */
@@ -2464,7 +2466,7 @@ typedef struct MinMaxAggInfo
  * for conflicting purposes.
  *
  * In addition, PARAM_EXEC slots are assigned for Params representing outputs
- * from subplans (values that are setParam items for those subplans).  These
+ * from subplans (values that are setParam items for those subplans). [TODO: is this true, or only for init plans?]  These
  * IDs need not be tracked via PlannerParamItems, since we do not need any
  * duplicate-elimination nor later processing of the represented expressions.
  * Instead, we just record the assignment of the slot number by appending to
@@ -2583,6 +2585,7 @@ typedef struct
 
 	/* Data which may differ across partitions. */
 	bool		target_parallel_safe;
+	bool		target_parallel_safe_ignoring_params;
 	Node	   *havingQual;
 	List	   *targetList;
 	PartitionwiseAggregateType patype;
diff --git a/src/include/nodes/plannodes.h b/src/include/nodes/plannodes.h
index d671328dfd..b36def462b 100644
--- a/src/include/nodes/plannodes.h
+++ b/src/include/nodes/plannodes.h
@@ -128,6 +128,7 @@ typedef struct Plan
 	 */
 	bool		parallel_aware; /* engage parallel-aware logic? */
 	bool		parallel_safe;	/* OK to use as part of parallel plan? */
+	bool		parallel_safe_ignoring_params;	/* OK to use as part of parallel plan if worker context provides params? */
 
 	/*
 	 * information needed for asynchronous execution
diff --git a/src/include/nodes/primnodes.h b/src/include/nodes/primnodes.h
index 9ae851d847..279a8e106b 100644
--- a/src/include/nodes/primnodes.h
+++ b/src/include/nodes/primnodes.h
@@ -754,6 +754,7 @@ typedef struct SubPlan
 								 * spec result is UNKNOWN; this allows much
 								 * simpler handling of null values */
 	bool		parallel_safe;	/* is the subplan parallel-safe? */
+	bool		parallel_safe_ignoring_params;	/* is the subplan parallel-safe when params are provided by the worker context? */
 	/* Note: parallel_safe does not consider contents of testexpr or args */
 	/* Information for passing params into and out of the subselect: */
 	/* setParam and parParam are lists of integers (param IDs) */
diff --git a/src/include/optimizer/clauses.h b/src/include/optimizer/clauses.h
index 0673887a85..df01be2c61 100644
--- a/src/include/optimizer/clauses.h
+++ b/src/include/optimizer/clauses.h
@@ -16,6 +16,8 @@
 
 #include "nodes/pathnodes.h"
 
+extern PGDLLIMPORT bool enable_parallel_params_recheck;
+
 typedef struct
 {
 	int			numWindowFuncs; /* total number of WindowFuncs found */
@@ -33,7 +35,7 @@ extern double expression_returns_set_rows(PlannerInfo *root, Node *clause);
 extern bool contain_subplans(Node *clause);
 
 extern char max_parallel_hazard(Query *parse);
-extern bool is_parallel_safe(PlannerInfo *root, Node *node);
+extern bool is_parallel_safe(PlannerInfo *root, Node *node, bool *safe_ignoring_params);
 extern bool contain_nonstrict_functions(Node *clause);
 extern bool contain_exec_param(Node *clause, List *param_ids);
 extern bool contain_leaked_vars(Node *clause);
diff --git a/src/test/regress/expected/incremental_sort.out b/src/test/regress/expected/incremental_sort.out
index 545e301e48..8f9ca05e60 100644
--- a/src/test/regress/expected/incremental_sort.out
+++ b/src/test/regress/expected/incremental_sort.out
@@ -1614,16 +1614,16 @@ from tenk1 t, generate_series(1, 1000);
                                    QUERY PLAN                                    
 ---------------------------------------------------------------------------------
  Unique
-   ->  Sort
-         Sort Key: t.unique1, ((SubPlan 1))
-         ->  Gather
-               Workers Planned: 2
+   ->  Gather Merge
+         Workers Planned: 2
+         ->  Sort
+               Sort Key: t.unique1, ((SubPlan 1))
                ->  Nested Loop
                      ->  Parallel Index Only Scan using tenk1_unique1 on tenk1 t
                      ->  Function Scan on generate_series
-               SubPlan 1
-                 ->  Index Only Scan using tenk1_unique1 on tenk1
-                       Index Cond: (unique1 = t.unique1)
+                     SubPlan 1
+                       ->  Index Only Scan using tenk1_unique1 on tenk1
+                             Index Cond: (unique1 = t.unique1)
 (11 rows)
 
 explain (costs off) select
@@ -1633,16 +1633,16 @@ from tenk1 t, generate_series(1, 1000)
 order by 1, 2;
                                 QUERY PLAN                                 
 ---------------------------------------------------------------------------
- Sort
-   Sort Key: t.unique1, ((SubPlan 1))
-   ->  Gather
-         Workers Planned: 2
+ Gather Merge
+   Workers Planned: 2
+   ->  Sort
+         Sort Key: t.unique1, ((SubPlan 1))
          ->  Nested Loop
                ->  Parallel Index Only Scan using tenk1_unique1 on tenk1 t
                ->  Function Scan on generate_series
-         SubPlan 1
-           ->  Index Only Scan using tenk1_unique1 on tenk1
-                 Index Cond: (unique1 = t.unique1)
+               SubPlan 1
+                 ->  Index Only Scan using tenk1_unique1 on tenk1
+                       Index Cond: (unique1 = t.unique1)
 (10 rows)
 
 -- Parallel sort but with expression not available until the upper rel.
diff --git a/src/test/regress/expected/select_parallel.out b/src/test/regress/expected/select_parallel.out
index 224a1da2af..e6f2453a1f 100644
--- a/src/test/regress/expected/select_parallel.out
+++ b/src/test/regress/expected/select_parallel.out
@@ -311,6 +311,131 @@ select count(*) from tenk1 where (two, four) not in
  10000
 (1 row)
 
+-- test parallel plans for queries containing correlated subplans
+-- where the subplan only needs params available from the current
+-- worker's scan.
+explain (costs off, verbose) select
+  (select t.unique1 from tenk1 where tenk1.unique1 = t.unique1)
+  from tenk1 t, generate_series(1, 10);
+                                 QUERY PLAN                                 
+----------------------------------------------------------------------------
+ Gather
+   Output: ((SubPlan 1))
+   Workers Planned: 4
+   ->  Nested Loop
+         Output: (SubPlan 1)
+         ->  Parallel Index Only Scan using tenk1_unique1 on public.tenk1 t
+               Output: t.unique1
+         ->  Function Scan on pg_catalog.generate_series
+               Output: generate_series.generate_series
+               Function Call: generate_series(1, 10)
+         SubPlan 1
+           ->  Index Only Scan using tenk1_unique1 on public.tenk1
+                 Output: t.unique1
+                 Index Cond: (tenk1.unique1 = t.unique1)
+(14 rows)
+
+explain (costs off, verbose) select
+  (select t.unique1 from tenk1 where tenk1.unique1 = t.unique1)
+  from tenk1 t;
+                              QUERY PLAN                              
+----------------------------------------------------------------------
+ Gather
+   Output: ((SubPlan 1))
+   Workers Planned: 4
+   ->  Parallel Index Only Scan using tenk1_unique1 on public.tenk1 t
+         Output: (SubPlan 1)
+         SubPlan 1
+           ->  Index Only Scan using tenk1_unique1 on public.tenk1
+                 Output: t.unique1
+                 Index Cond: (tenk1.unique1 = t.unique1)
+(9 rows)
+
+explain (analyze, costs off, summary off, verbose, timing off) select
+  (select t.unique1 from tenk1 where tenk1.unique1 = t.unique1)
+  from tenk1 t
+  limit 1;
+                                             QUERY PLAN                                             
+----------------------------------------------------------------------------------------------------
+ Limit (actual rows=1 loops=1)
+   Output: ((SubPlan 1))
+   ->  Gather (actual rows=1 loops=1)
+         Output: ((SubPlan 1))
+         Workers Planned: 4
+         Workers Launched: 4
+         ->  Parallel Index Only Scan using tenk1_unique1 on public.tenk1 t (actual rows=1 loops=5)
+               Output: (SubPlan 1)
+               Heap Fetches: 0
+               Worker 0:  actual rows=1 loops=1
+               Worker 1:  actual rows=1 loops=1
+               Worker 2:  actual rows=1 loops=1
+               Worker 3:  actual rows=1 loops=1
+               SubPlan 1
+                 ->  Index Only Scan using tenk1_unique1 on public.tenk1 (actual rows=1 loops=5)
+                       Output: t.unique1
+                       Index Cond: (tenk1.unique1 = t.unique1)
+                       Heap Fetches: 0
+                       Worker 0:  actual rows=1 loops=1
+                       Worker 1:  actual rows=1 loops=1
+                       Worker 2:  actual rows=1 loops=1
+                       Worker 3:  actual rows=1 loops=1
+(22 rows)
+
+explain (costs off, verbose) select t.unique1
+  from tenk1 t
+  where t.unique1 = (select t.unique1 from tenk1 where tenk1.unique1 = t.unique1);
+                              QUERY PLAN                              
+----------------------------------------------------------------------
+ Gather
+   Output: t.unique1
+   Workers Planned: 4
+   ->  Parallel Index Only Scan using tenk1_unique1 on public.tenk1 t
+         Output: t.unique1
+         Filter: (t.unique1 = (SubPlan 1))
+         SubPlan 1
+           ->  Index Only Scan using tenk1_unique1 on public.tenk1
+                 Output: t.unique1
+                 Index Cond: (tenk1.unique1 = t.unique1)
+(10 rows)
+
+explain (costs off, verbose) select *
+  from tenk1 t
+  order by (select t.unique1 from tenk1 where tenk1.unique1 = t.unique1);
+                                                                                                QUERY PLAN                                                                                                
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
+ Gather Merge
+   Output: t.unique1, t.unique2, t.two, t.four, t.ten, t.twenty, t.hundred, t.thousand, t.twothousand, t.fivethous, t.tenthous, t.odd, t.even, t.stringu1, t.stringu2, t.string4, ((SubPlan 1))
+   Workers Planned: 4
+   ->  Sort
+         Output: t.unique1, t.unique2, t.two, t.four, t.ten, t.twenty, t.hundred, t.thousand, t.twothousand, t.fivethous, t.tenthous, t.odd, t.even, t.stringu1, t.stringu2, t.string4, ((SubPlan 1))
+         Sort Key: ((SubPlan 1))
+         ->  Parallel Seq Scan on public.tenk1 t
+               Output: t.unique1, t.unique2, t.two, t.four, t.ten, t.twenty, t.hundred, t.thousand, t.twothousand, t.fivethous, t.tenthous, t.odd, t.even, t.stringu1, t.stringu2, t.string4, (SubPlan 1)
+               SubPlan 1
+                 ->  Index Only Scan using tenk1_unique1 on public.tenk1
+                       Output: t.unique1
+                       Index Cond: (tenk1.unique1 = t.unique1)
+(12 rows)
+
+-- test subplan in join/lateral join
+explain (costs off, verbose, timing off) select t.unique1, l.*
+  from tenk1 t
+  join lateral (
+    select (select t.unique1 from tenk1 where tenk1.unique1 = t.unique1 offset 0)
+  ) l on true;
+                              QUERY PLAN                              
+----------------------------------------------------------------------
+ Gather
+   Output: t.unique1, ((SubPlan 1))
+   Workers Planned: 4
+   ->  Parallel Index Only Scan using tenk1_unique1 on public.tenk1 t
+         Output: t.unique1, (SubPlan 1)
+         SubPlan 1
+           ->  Index Only Scan using tenk1_unique1 on public.tenk1
+                 Output: t.unique1
+                 Index Cond: (tenk1.unique1 = t.unique1)
+(9 rows)
+
 -- this is not parallel-safe due to use of random() within SubLink's testexpr:
 explain (costs off)
 	select * from tenk1 where (unique1 + random())::integer not in
@@ -1169,6 +1294,18 @@ EXECUTE pstmt('1', make_some_array(1,2));
 
 DEALLOCATE pstmt;
 -- test interaction between subquery and partial_paths
+-- this plan changes to using a non-parallel index only
+-- scan on tenk1_unique1 (the parallel version of the subquery scan
+-- is cheaper, but only by ~30, and cost comparison treats them as equal
+-- since the costs are so large) because set_rel_consider_parallel
+-- called from make_one_rel sees the subplan as parallel safe now
+-- (in context it now knows the params are actually parallel safe).
+-- Because of that the non-parallel index path is now parallel_safe=true,
+-- therefore it wins the COSTS_EQUAL comparison in add_path.
+-- Perhaps any is_parallel_safe calls made for the purpose of determining
+-- consider_parallel should disable that behavior? It's not clear which is
+-- correct.
+set enable_parallel_params_recheck = off;
 CREATE VIEW tenk1_vw_sec WITH (security_barrier) AS SELECT * FROM tenk1;
 EXPLAIN (COSTS OFF)
 SELECT 1 FROM tenk1_vw_sec
diff --git a/src/test/regress/expected/sysviews.out b/src/test/regress/expected/sysviews.out
index 0bb558d93c..22fee51455 100644
--- a/src/test/regress/expected/sysviews.out
+++ b/src/test/regress/expected/sysviews.out
@@ -108,6 +108,7 @@ select name, setting from pg_settings where name like 'enable%';
  enable_nestloop                | on
  enable_parallel_append         | on
  enable_parallel_hash           | on
+ enable_parallel_params_recheck | on
  enable_partition_pruning       | on
  enable_partitionwise_aggregate | off
  enable_partitionwise_join      | off
@@ -115,7 +116,7 @@ select name, setting from pg_settings where name like 'enable%';
  enable_seqscan                 | on
  enable_sort                    | on
  enable_tidscan                 | on
-(20 rows)
+(21 rows)
 
 -- Test that the pg_timezone_names and pg_timezone_abbrevs views are
 -- more-or-less working.  We can't test their contents in any great detail
diff --git a/src/test/regress/sql/select_parallel.sql b/src/test/regress/sql/select_parallel.sql
index f656764fa9..55a3bd9025 100644
--- a/src/test/regress/sql/select_parallel.sql
+++ b/src/test/regress/sql/select_parallel.sql
@@ -111,6 +111,31 @@ explain (costs off)
 	(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);
+-- test parallel plans for queries containing correlated subplans
+-- where the subplan only needs params available from the current
+-- worker's scan.
+explain (costs off, verbose) select
+  (select t.unique1 from tenk1 where tenk1.unique1 = t.unique1)
+  from tenk1 t, generate_series(1, 10);
+explain (costs off, verbose) select
+  (select t.unique1 from tenk1 where tenk1.unique1 = t.unique1)
+  from tenk1 t;
+explain (analyze, costs off, summary off, verbose, timing off) select
+  (select t.unique1 from tenk1 where tenk1.unique1 = t.unique1)
+  from tenk1 t
+  limit 1;
+explain (costs off, verbose) select t.unique1
+  from tenk1 t
+  where t.unique1 = (select t.unique1 from tenk1 where tenk1.unique1 = t.unique1);
+explain (costs off, verbose) select *
+  from tenk1 t
+  order by (select t.unique1 from tenk1 where tenk1.unique1 = t.unique1);
+-- test subplan in join/lateral join
+explain (costs off, verbose, timing off) select t.unique1, l.*
+  from tenk1 t
+  join lateral (
+    select (select t.unique1 from tenk1 where tenk1.unique1 = t.unique1 offset 0)
+  ) l on true;
 -- this is not parallel-safe due to use of random() within SubLink's testexpr:
 explain (costs off)
 	select * from tenk1 where (unique1 + random())::integer not in
@@ -452,6 +477,18 @@ EXECUTE pstmt('1', make_some_array(1,2));
 DEALLOCATE pstmt;
 
 -- test interaction between subquery and partial_paths
+-- this plan changes to using a non-parallel index only
+-- scan on tenk1_unique1 (the parallel version of the subquery scan
+-- is cheaper, but only by ~30, and cost comparison treats them as equal
+-- since the costs are so large) because set_rel_consider_parallel
+-- called from make_one_rel sees the subplan as parallel safe now
+-- (in context it now knows the params are actually parallel safe).
+-- Because of that the non-parallel index path is now parallel_safe=true,
+-- therefore it wins the COSTS_EQUAL comparison in add_path.
+-- Perhaps any is_parallel_safe calls made for the purpose of determining
+-- consider_parallel should disable that behavior? It's not clear which is
+-- correct.
+set enable_parallel_params_recheck = off;
 CREATE VIEW tenk1_vw_sec WITH (security_barrier) AS SELECT * FROM tenk1;
 EXPLAIN (COSTS OFF)
 SELECT 1 FROM tenk1_vw_sec
-- 
2.20.1