Don't consider parallel append for parallel unsafe paths.

Commit ab727167 allowed Parallel Append paths to be generated for a
relation that is not parallel safe.  Prevent that from happening.

Initial analysis by Tom Lane.

Reported-by: Rajkumar Raghuwanshi
Author: Amit Kapila and Rajkumar Raghuwanshi
Reviewed-by: Amit Khandekar and Robert Haas
Discussion:https://postgr.es/m/CAKcux6=tPJ6nJ08r__nU_pmLQiC0xY15Fn0HvG1Cprsjdd9s_Q@mail.gmail.com

src/backend/optimizer/path/allpaths.c

@@ -1383,7 +1383,7 @@ add_paths_to_append_rel(PlannerInfo *root, RelOptInfo *rel,
 	List	   *pa_partial_subpaths = NIL;
 	List	   *pa_nonpartial_subpaths = NIL;
 	bool		partial_subpaths_valid = true;
-	bool		pa_subpaths_valid = enable_parallel_append;
+	bool		pa_subpaths_valid;
 	List	   *all_child_pathkeys = NIL;
 	List	   *all_child_outers = NIL;
 	ListCell   *l;
@@ -1391,6 +1391,9 @@ add_paths_to_append_rel(PlannerInfo *root, RelOptInfo *rel,
 	bool		build_partitioned_rels = false;
 	double		partial_rows = -1;
 
+	/* If appropriate, consider parallel append */
+	pa_subpaths_valid = enable_parallel_append && rel->consider_parallel;
+
 	/*
 	 * AppendPath generated for partitioned tables must record the RT indexes
 	 * of partitioned tables that are direct or indirect children of this

src/test/regress/expected/select_parallel.out

@@ -132,6 +132,32 @@ select sp_test_func() order by 1;
  foo
 (2 rows)
 
+-- Parallel Append is not to be used when the subpath depends on the outer param
+create table part_pa_test(a int, b int) partition by range(a);
+create table part_pa_test_p1 partition of part_pa_test for values from (minvalue) to (0);
+create table part_pa_test_p2 partition of part_pa_test for values from (0) to (maxvalue);
+explain (costs off)
+	select (select max((select pa1.b from part_pa_test pa1 where pa1.a = pa2.a)))
+	from part_pa_test pa2;
+                          QUERY PLAN                          
+--------------------------------------------------------------
+ Aggregate
+   ->  Gather
+         Workers Planned: 3
+         ->  Parallel Append
+               ->  Parallel Seq Scan on part_pa_test_p1 pa2
+               ->  Parallel Seq Scan on part_pa_test_p2 pa2_1
+   SubPlan 2
+     ->  Result
+   SubPlan 1
+     ->  Append
+           ->  Seq Scan on part_pa_test_p1 pa1
+                 Filter: (a = pa2.a)
+           ->  Seq Scan on part_pa_test_p2 pa1_1
+                 Filter: (a = pa2.a)
+(14 rows)
+
+drop table part_pa_test;
 -- test with leader participation disabled
 set parallel_leader_participation = off;
 explain (costs off)

src/test/regress/sql/select_parallel.sql

@@ -55,6 +55,15 @@ $$ select 'foo'::varchar union all select 'bar'::varchar $$
 language sql stable;
 select sp_test_func() order by 1;
 
+-- Parallel Append is not to be used when the subpath depends on the outer param
+create table part_pa_test(a int, b int) partition by range(a);
+create table part_pa_test_p1 partition of part_pa_test for values from (minvalue) to (0);
+create table part_pa_test_p2 partition of part_pa_test for values from (0) to (maxvalue);
+explain (costs off)
+	select (select max((select pa1.b from part_pa_test pa1 where pa1.a = pa2.a)))
+	from part_pa_test pa2;
+drop table part_pa_test;
+
 -- test with leader participation disabled
 set parallel_leader_participation = off;
 explain (costs off)