Check parallel safety in generate_useful_gather_paths

Commit ebb7ae83 ensured we ignore pathkeys with volatile expressions
when considering adding a sort below a Gather Merge. Turns out we need
to care about parallel safety of the pathkeys too, otherwise we might
try sorting e.g. on results of a correlated subquery (as demonstrated
by a report from Luis Roberto).

Initial investigation by Tom Lane, patch by James Coleman. Backpatch
to 13, where the code was instroduced (as part of Incremental Sort).

Reported-by: Luis Roberto
Author: James Coleman
Reviewed-by: Tomas Vondra
Backpatch-through: 13
Discussion: https://postgr.es/m/622580997.37108180.1604080457319.JavaMail.zimbra%40siscobra.com.br
Discussion: https://postgr.es/m/CAAaqYe8cK3g5CfLC4w7bs=hC0mSksZC=H5M8LSchj5e5OxpTAg@mail.gmail.com

src/backend/optimizer/path/allpaths.c

@@ -2802,6 +2802,9 @@ generate_gather_paths(PlannerInfo *root, RelOptInfo *rel, bool override_rows)
  * This allows us to do incremental sort on top of an index scan under a gather
  * merge node, i.e. parallelized.
  *
+ * If the require_parallel_safe is true, we also require the expressions to
+ * be parallel safe (which allows pushing the sort below Gather Merge).
+ *
  * XXX At the moment this can only ever return a list with a single element,
  * because it looks at query_pathkeys only. So we might return the pathkeys
  * directly, but it seems plausible we'll want to consider other orderings
@@ -2809,7 +2812,8 @@ generate_gather_paths(PlannerInfo *root, RelOptInfo *rel, bool override_rows)
  * merge joins.
  */
 static List *
-get_useful_pathkeys_for_relation(PlannerInfo *root, RelOptInfo *rel)
+get_useful_pathkeys_for_relation(PlannerInfo *root, RelOptInfo *rel,
+								 bool require_parallel_safe)
 {
 	List	   *useful_pathkeys_list = NIL;
 
@@ -2839,8 +2843,11 @@ get_useful_pathkeys_for_relation(PlannerInfo *root, RelOptInfo *rel)
 			 * meet criteria of EC membership in the current relation, we
 			 * enable not just an incremental sort on the entirety of
 			 * query_pathkeys but also incremental sort below a JOIN.
+			 *
+			 * If requested, ensure the expression is parallel safe too.
 			 */
-			if (!find_em_expr_usable_for_sorting_rel(pathkey_ec, rel))
+			if (!find_em_expr_usable_for_sorting_rel(root, pathkey_ec, rel,
+													 require_parallel_safe))
 				break;
 
 			npathkeys++;
@@ -2894,7 +2901,7 @@ generate_useful_gather_paths(PlannerInfo *root, RelOptInfo *rel, bool override_r
 	generate_gather_paths(root, rel, override_rows);
 
 	/* consider incremental sort for interesting orderings */
-	useful_pathkeys_list = get_useful_pathkeys_for_relation(root, rel);
+	useful_pathkeys_list = get_useful_pathkeys_for_relation(root, rel, true);
 
 	/* used for explicit (full) sort paths */
 	cheapest_partial_path = linitial(rel->partial_pathlist);

src/backend/optimizer/path/equivclass.c

@@ -803,7 +803,8 @@ find_em_expr_for_rel(EquivalenceClass *ec, RelOptInfo *rel)
  * applied in prepare_sort_from_pathkeys.
  */
 Expr *
-find_em_expr_usable_for_sorting_rel(EquivalenceClass *ec, RelOptInfo *rel)
+find_em_expr_usable_for_sorting_rel(PlannerInfo *root, EquivalenceClass *ec,
+									RelOptInfo *rel, bool require_parallel_safe)
 {
 	ListCell   *lc_em;
 
@@ -833,6 +834,12 @@ find_em_expr_usable_for_sorting_rel(EquivalenceClass *ec, RelOptInfo *rel)
 		if (!bms_is_subset(em->em_relids, rel->relids))
 			continue;
 
+		/*
+		 * If requested, reject expressions that are not parallel-safe.
+		 */
+		if (require_parallel_safe && !is_parallel_safe(root, (Node *) em_expr))
+			continue;
+
 		/*
 		 * As long as the expression isn't volatile then
 		 * prepare_sort_from_pathkeys is able to generate a new target entry,

src/include/optimizer/paths.h

@@ -135,7 +135,10 @@ extern EquivalenceClass *get_eclass_for_sort_expr(PlannerInfo *root,
 												  Relids rel,
 												  bool create_it);
 extern Expr *find_em_expr_for_rel(EquivalenceClass *ec, RelOptInfo *rel);
-extern Expr *find_em_expr_usable_for_sorting_rel(EquivalenceClass *ec, RelOptInfo *rel);
+extern Expr *find_em_expr_usable_for_sorting_rel(PlannerInfo *root,
+												 EquivalenceClass *ec,
+												 RelOptInfo *rel,
+												 bool require_parallel_safe);
 extern void generate_base_implied_equalities(PlannerInfo *root);
 extern List *generate_join_implied_equalities(PlannerInfo *root,
 											  Relids join_relids,

src/test/regress/expected/incremental_sort.out

@@ -1551,6 +1551,46 @@ order by 1, 2;
    ->  Function Scan on generate_series
 (7 rows)
 
+-- Parallel sort but with expression (correlated subquery) that
+-- is prohibited in parallel plans.
+explain (costs off) select distinct
+  unique1,
+  (select t.unique1 from tenk1 where tenk1.unique1 = t.unique1)
+from tenk1 t, generate_series(1, 1000);
+                                   QUERY PLAN                                    
+---------------------------------------------------------------------------------
+ Unique
+   ->  Sort
+         Sort Key: t.unique1, ((SubPlan 1))
+         ->  Gather
+               Workers Planned: 2
+               ->  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)
+(11 rows)
+
+explain (costs off) select
+  unique1,
+  (select t.unique1 from tenk1 where tenk1.unique1 = t.unique1)
+from tenk1 t, generate_series(1, 1000)
+order by 1, 2;
+                                QUERY PLAN                                 
+---------------------------------------------------------------------------
+ Sort
+   Sort Key: t.unique1, ((SubPlan 1))
+   ->  Gather
+         Workers Planned: 2
+         ->  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)
+(10 rows)
+
 -- Parallel sort but with expression not available until the upper rel.
 explain (costs off) select distinct sub.unique1, stringu1 || random()::text
 from tenk1, lateral (select tenk1.unique1 from generate_series(1, 1000)) as sub;

src/test/regress/sql/incremental_sort.sql

@@ -250,6 +250,17 @@ from tenk1, lateral (select tenk1.unique1 from generate_series(1, 1000)) as sub;
 explain (costs off) select sub.unique1, md5(stringu1)
 from tenk1, lateral (select tenk1.unique1 from generate_series(1, 1000)) as sub
 order by 1, 2;
+-- Parallel sort but with expression (correlated subquery) that
+-- is prohibited in parallel plans.
+explain (costs off) select distinct
+  unique1,
+  (select t.unique1 from tenk1 where tenk1.unique1 = t.unique1)
+from tenk1 t, generate_series(1, 1000);
+explain (costs off) select
+  unique1,
+  (select t.unique1 from tenk1 where tenk1.unique1 = t.unique1)
+from tenk1 t, generate_series(1, 1000)
+order by 1, 2;
 -- Parallel sort but with expression not available until the upper rel.
 explain (costs off) select distinct sub.unique1, stringu1 || random()::text
 from tenk1, lateral (select tenk1.unique1 from generate_series(1, 1000)) as sub;