Check parallel safety in generate_useful_gather_paths

Commit ebb7ae839d 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

2 files changed, 51 insertions, 0 deletions

diff --git a/src/test/regress/expected/incremental_sort.out b/src/test/regress/expected/incremental_sort.out
index 51471ae92de..d316a7c4b90 100644
--- a/src/test/regress/expected/incremental_sort.out
+++ b/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;
diff --git a/src/test/regress/sql/incremental_sort.sql b/src/test/regress/sql/incremental_sort.sql
index cb48f200ce5..ff3af0fd165 100644
--- a/src/test/regress/sql/incremental_sort.sql
+++ b/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;