Force parallelism in partition_aggregate

Commit db0d67db2 tweaked sort costing, which however resulted in a
couple plan changes in our regression tests. Most of the new plans were
fine, but partition_aggregate were meant to test parallel plans and the
new plans were serial.

Fix that by lowering parallel_setup_cost to 0, which is enough to switch
to the parallel plan again.

Report and patch by David Rowley.

Author: David Rowley
Reviewed-by: Tomas Vondra
Discussion: https://postgr.es/m/CAApHDvpVFgWzXdtUQkjyOPhNrNvumRi_=ftgS79KeAZ92tnHKQ@mail.gmail.com

2 files changed, 63 insertions, 54 deletions

diff --git a/src/test/regress/expected/partition_aggregate.out b/src/test/regress/expected/partition_aggregate.out
index a08a3825ff6..0dc6d633479 100644
--- a/src/test/regress/expected/partition_aggregate.out
+++ b/src/test/regress/expected/partition_aggregate.out
@@ -942,40 +942,43 @@ INSERT INTO pagg_tab_ml SELECT i % 30, i % 10, to_char(i % 4, 'FM0000') FROM gen
 ANALYZE pagg_tab_ml;
 -- For Parallel Append
 SET max_parallel_workers_per_gather TO 2;
+SET parallel_setup_cost = 0;
 -- Full aggregation at level 1 as GROUP BY clause matches with PARTITION KEY
 -- for level 1 only. For subpartitions, GROUP BY clause does not match with
 -- PARTITION KEY, but still we do not see a partial aggregation as array_agg()
 -- is not partial agg safe.
 EXPLAIN (COSTS OFF)
 SELECT a, sum(b), array_agg(distinct c), count(*) FROM pagg_tab_ml GROUP BY a HAVING avg(b) < 3 ORDER BY 1, 2, 3;
-                                      QUERY PLAN                                      
---------------------------------------------------------------------------------------
- Sort
-   Sort Key: pagg_tab_ml.a, (sum(pagg_tab_ml.b)), (array_agg(DISTINCT pagg_tab_ml.c))
-   ->  Append
-         ->  GroupAggregate
-               Group Key: pagg_tab_ml.a
-               Filter: (avg(pagg_tab_ml.b) < '3'::numeric)
-               ->  Sort
-                     Sort Key: pagg_tab_ml.a
-                     ->  Seq Scan on pagg_tab_ml_p1 pagg_tab_ml
-         ->  GroupAggregate
-               Group Key: pagg_tab_ml_2.a
-               Filter: (avg(pagg_tab_ml_2.b) < '3'::numeric)
-               ->  Sort
-                     Sort Key: pagg_tab_ml_2.a
-                     ->  Append
-                           ->  Seq Scan on pagg_tab_ml_p2_s1 pagg_tab_ml_2
-                           ->  Seq Scan on pagg_tab_ml_p2_s2 pagg_tab_ml_3
-         ->  GroupAggregate
-               Group Key: pagg_tab_ml_5.a
-               Filter: (avg(pagg_tab_ml_5.b) < '3'::numeric)
-               ->  Sort
-                     Sort Key: pagg_tab_ml_5.a
-                     ->  Append
-                           ->  Seq Scan on pagg_tab_ml_p3_s1 pagg_tab_ml_5
-                           ->  Seq Scan on pagg_tab_ml_p3_s2 pagg_tab_ml_6
-(25 rows)
+                                         QUERY PLAN                                         
+--------------------------------------------------------------------------------------------
+ Gather Merge
+   Workers Planned: 2
+   ->  Sort
+         Sort Key: pagg_tab_ml.a, (sum(pagg_tab_ml.b)), (array_agg(DISTINCT pagg_tab_ml.c))
+         ->  Parallel Append
+               ->  GroupAggregate
+                     Group Key: pagg_tab_ml.a
+                     Filter: (avg(pagg_tab_ml.b) < '3'::numeric)
+                     ->  Sort
+                           Sort Key: pagg_tab_ml.a
+                           ->  Seq Scan on pagg_tab_ml_p1 pagg_tab_ml
+               ->  GroupAggregate
+                     Group Key: pagg_tab_ml_5.a
+                     Filter: (avg(pagg_tab_ml_5.b) < '3'::numeric)
+                     ->  Sort
+                           Sort Key: pagg_tab_ml_5.a
+                           ->  Append
+                                 ->  Seq Scan on pagg_tab_ml_p3_s1 pagg_tab_ml_5
+                                 ->  Seq Scan on pagg_tab_ml_p3_s2 pagg_tab_ml_6
+               ->  GroupAggregate
+                     Group Key: pagg_tab_ml_2.a
+                     Filter: (avg(pagg_tab_ml_2.b) < '3'::numeric)
+                     ->  Sort
+                           Sort Key: pagg_tab_ml_2.a
+                           ->  Append
+                                 ->  Seq Scan on pagg_tab_ml_p2_s1 pagg_tab_ml_2
+                                 ->  Seq Scan on pagg_tab_ml_p2_s2 pagg_tab_ml_3
+(27 rows)
 
 SELECT a, sum(b), array_agg(distinct c), count(*) FROM pagg_tab_ml GROUP BY a HAVING avg(b) < 3 ORDER BY 1, 2, 3;
  a  | sum  |  array_agg  | count 
@@ -994,33 +997,36 @@ SELECT a, sum(b), array_agg(distinct c), count(*) FROM pagg_tab_ml GROUP BY a HA
 -- Without ORDER BY clause, to test Gather at top-most path
 EXPLAIN (COSTS OFF)
 SELECT a, sum(b), array_agg(distinct c), count(*) FROM pagg_tab_ml GROUP BY a HAVING avg(b) < 3;
-                             QUERY PLAN                              
----------------------------------------------------------------------
- Append
-   ->  GroupAggregate
-         Group Key: pagg_tab_ml.a
-         Filter: (avg(pagg_tab_ml.b) < '3'::numeric)
-         ->  Sort
-               Sort Key: pagg_tab_ml.a
-               ->  Seq Scan on pagg_tab_ml_p1 pagg_tab_ml
-   ->  GroupAggregate
-         Group Key: pagg_tab_ml_2.a
-         Filter: (avg(pagg_tab_ml_2.b) < '3'::numeric)
-         ->  Sort
-               Sort Key: pagg_tab_ml_2.a
-               ->  Append
-                     ->  Seq Scan on pagg_tab_ml_p2_s1 pagg_tab_ml_2
-                     ->  Seq Scan on pagg_tab_ml_p2_s2 pagg_tab_ml_3
-   ->  GroupAggregate
-         Group Key: pagg_tab_ml_5.a
-         Filter: (avg(pagg_tab_ml_5.b) < '3'::numeric)
-         ->  Sort
-               Sort Key: pagg_tab_ml_5.a
-               ->  Append
-                     ->  Seq Scan on pagg_tab_ml_p3_s1 pagg_tab_ml_5
-                     ->  Seq Scan on pagg_tab_ml_p3_s2 pagg_tab_ml_6
-(23 rows)
+                                QUERY PLAN                                 
+---------------------------------------------------------------------------
+ Gather
+   Workers Planned: 2
+   ->  Parallel Append
+         ->  GroupAggregate
+               Group Key: pagg_tab_ml.a
+               Filter: (avg(pagg_tab_ml.b) < '3'::numeric)
+               ->  Sort
+                     Sort Key: pagg_tab_ml.a
+                     ->  Seq Scan on pagg_tab_ml_p1 pagg_tab_ml
+         ->  GroupAggregate
+               Group Key: pagg_tab_ml_5.a
+               Filter: (avg(pagg_tab_ml_5.b) < '3'::numeric)
+               ->  Sort
+                     Sort Key: pagg_tab_ml_5.a
+                     ->  Append
+                           ->  Seq Scan on pagg_tab_ml_p3_s1 pagg_tab_ml_5
+                           ->  Seq Scan on pagg_tab_ml_p3_s2 pagg_tab_ml_6
+         ->  GroupAggregate
+               Group Key: pagg_tab_ml_2.a
+               Filter: (avg(pagg_tab_ml_2.b) < '3'::numeric)
+               ->  Sort
+                     Sort Key: pagg_tab_ml_2.a
+                     ->  Append
+                           ->  Seq Scan on pagg_tab_ml_p2_s1 pagg_tab_ml_2
+                           ->  Seq Scan on pagg_tab_ml_p2_s2 pagg_tab_ml_3
+(25 rows)
 
+RESET parallel_setup_cost;
 -- Full aggregation at level 1 as GROUP BY clause matches with PARTITION KEY
 -- for level 1 only. For subpartitions, GROUP BY clause does not match with
 -- PARTITION KEY, thus we will have a partial aggregation for them.
diff --git a/src/test/regress/sql/partition_aggregate.sql b/src/test/regress/sql/partition_aggregate.sql
index c17294b15b0..ab070fee244 100644
--- a/src/test/regress/sql/partition_aggregate.sql
+++ b/src/test/regress/sql/partition_aggregate.sql
@@ -222,6 +222,7 @@ ANALYZE pagg_tab_ml;
 
 -- For Parallel Append
 SET max_parallel_workers_per_gather TO 2;
+SET parallel_setup_cost = 0;
 
 -- Full aggregation at level 1 as GROUP BY clause matches with PARTITION KEY
 -- for level 1 only. For subpartitions, GROUP BY clause does not match with
@@ -235,6 +236,8 @@ SELECT a, sum(b), array_agg(distinct c), count(*) FROM pagg_tab_ml GROUP BY a HA
 EXPLAIN (COSTS OFF)
 SELECT a, sum(b), array_agg(distinct c), count(*) FROM pagg_tab_ml GROUP BY a HAVING avg(b) < 3;
 
+RESET parallel_setup_cost;
+
 -- Full aggregation at level 1 as GROUP BY clause matches with PARTITION KEY
 -- for level 1 only. For subpartitions, GROUP BY clause does not match with
 -- PARTITION KEY, thus we will have a partial aggregation for them.