Attempt to stabilize partition_prune test output (2)

Environmental conditions might cause parallel workers to be scheduled in
different ways in this test, destabilizing the EXPLAIN output.  Disable
use of workers in an attempt to make output stable.

Author: David Rowley
Diagnosed-by: Thomas Munro
Discussion: https://postgr.es/m/CAKJS1f8j24tUX_nOwACiM=UO5jrMrDz8ca0xbG0vhVgfWph0ZA@mail.gmail.com

src/test/regress/expected/partition_prune.out

@@ -1563,7 +1563,10 @@ select avg(a) from ab where a between $1 and $2 and b < 4;
 set parallel_setup_cost = 0;
 set parallel_tuple_cost = 0;
 set min_parallel_table_scan_size = 0;
+-- set this so we get a parallel plan
 set max_parallel_workers_per_gather = 2;
+-- and zero this so that workers don't destabilize the explain output
+set max_parallel_workers = 0;
 -- Execute query 5 times to allow choose_custom_plan
 -- to start considering a generic plan.
 execute ab_q4 (1, 8);
@@ -1600,11 +1603,11 @@ explain (analyze, costs off, summary off, timing off) execute ab_q4 (2, 2);
                                   QUERY PLAN                                   
 -------------------------------------------------------------------------------
  Finalize Aggregate (actual rows=1 loops=1)
-   ->  Gather (actual rows=3 loops=1)
+   ->  Gather (actual rows=1 loops=1)
          Workers Planned: 2
-         Workers Launched: 2
-         ->  Partial Aggregate (actual rows=1 loops=3)
-               ->  Parallel Append (actual rows=0 loops=3)
+         Workers Launched: 0
+         ->  Partial Aggregate (actual rows=1 loops=1)
+               ->  Parallel Append (actual rows=0 loops=1)
                      Subplans Removed: 6
                      ->  Parallel Seq Scan on ab_a2_b1 (actual rows=0 loops=1)
                            Filter: ((a >= $1) AND (a <= $2) AND (b < 4))
@@ -1653,11 +1656,11 @@ explain (analyze, costs off, summary off, timing off) execute ab_q5 (1, 1, 1);
                                   QUERY PLAN                                   
 -------------------------------------------------------------------------------
  Finalize Aggregate (actual rows=1 loops=1)
-   ->  Gather (actual rows=3 loops=1)
+   ->  Gather (actual rows=1 loops=1)
          Workers Planned: 2
-         Workers Launched: 2
-         ->  Partial Aggregate (actual rows=1 loops=3)
-               ->  Parallel Append (actual rows=0 loops=3)
+         Workers Launched: 0
+         ->  Partial Aggregate (actual rows=1 loops=1)
+               ->  Parallel Append (actual rows=0 loops=1)
                      Subplans Removed: 6
                      ->  Parallel Seq Scan on ab_a1_b1 (actual rows=0 loops=1)
                            Filter: ((b < 4) AND (a = ANY (ARRAY[$1, $2, $3])))
@@ -1671,11 +1674,11 @@ explain (analyze, costs off, summary off, timing off) execute ab_q5 (2, 3, 3);
                                   QUERY PLAN                                   
 -------------------------------------------------------------------------------
  Finalize Aggregate (actual rows=1 loops=1)
-   ->  Gather (actual rows=3 loops=1)
+   ->  Gather (actual rows=1 loops=1)
          Workers Planned: 2
-         Workers Launched: 2
-         ->  Partial Aggregate (actual rows=1 loops=3)
-               ->  Parallel Append (actual rows=0 loops=3)
+         Workers Launched: 0
+         ->  Partial Aggregate (actual rows=1 loops=1)
+               ->  Parallel Append (actual rows=0 loops=1)
                      Subplans Removed: 3
                      ->  Parallel Seq Scan on ab_a2_b1 (actual rows=0 loops=1)
                            Filter: ((b < 4) AND (a = ANY (ARRAY[$1, $2, $3])))
@@ -1697,11 +1700,11 @@ explain (analyze, costs off, summary off, timing off) execute ab_q5 (33, 44, 55)
                                   QUERY PLAN                                   
 -------------------------------------------------------------------------------
  Finalize Aggregate (actual rows=1 loops=1)
-   ->  Gather (actual rows=3 loops=1)
+   ->  Gather (actual rows=1 loops=1)
          Workers Planned: 2
-         Workers Launched: 2
-         ->  Partial Aggregate (actual rows=1 loops=3)
-               ->  Parallel Append (actual rows=0 loops=3)
+         Workers Launched: 0
+         ->  Partial Aggregate (actual rows=1 loops=1)
+               ->  Parallel Append (actual rows=0 loops=1)
                      Subplans Removed: 8
                      ->  Parallel Seq Scan on ab_a1_b1 (never executed)
                            Filter: ((b < 4) AND (a = ANY (ARRAY[$1, $2, $3])))
@@ -1761,12 +1764,12 @@ explain (analyze, costs off, summary off, timing off) execute ab_q6 (0, 0, 1);
                                             QUERY PLAN                                             
 ---------------------------------------------------------------------------------------------------
  Finalize Aggregate (actual rows=1 loops=1)
-   ->  Gather (actual rows=2 loops=1)
+   ->  Gather (actual rows=1 loops=1)
          Workers Planned: 1
-         Workers Launched: 1
-         ->  Partial Aggregate (actual rows=1 loops=2)
-               ->  Nested Loop (actual rows=0 loops=2)
-                     ->  Parallel Seq Scan on lprt_a a (actual rows=51 loops=2)
+         Workers Launched: 0
+         ->  Partial Aggregate (actual rows=1 loops=1)
+               ->  Nested Loop (actual rows=0 loops=1)
+                     ->  Parallel Seq Scan on lprt_a a (actual rows=102 loops=1)
                            Filter: (a = ANY ('{0,0,1}'::integer[]))
                      ->  Append (actual rows=0 loops=102)
                            ->  Index Scan using ab_a1_b1_a_idx on ab_a1_b1 (actual rows=0 loops=2)
@@ -1794,12 +1797,12 @@ explain (analyze, costs off, summary off, timing off) execute ab_q6 (1, 0, 3);
                                             QUERY PLAN                                             
 ---------------------------------------------------------------------------------------------------
  Finalize Aggregate (actual rows=1 loops=1)
-   ->  Gather (actual rows=2 loops=1)
+   ->  Gather (actual rows=1 loops=1)
          Workers Planned: 1
-         Workers Launched: 1
-         ->  Partial Aggregate (actual rows=1 loops=2)
-               ->  Nested Loop (actual rows=0 loops=2)
-                     ->  Parallel Seq Scan on lprt_a a (actual rows=52 loops=2)
+         Workers Launched: 0
+         ->  Partial Aggregate (actual rows=1 loops=1)
+               ->  Nested Loop (actual rows=0 loops=1)
+                     ->  Parallel Seq Scan on lprt_a a (actual rows=104 loops=1)
                            Filter: (a = ANY ('{1,0,3}'::integer[]))
                      ->  Append (actual rows=0 loops=104)
                            ->  Index Scan using ab_a1_b1_a_idx on ab_a1_b1 (actual rows=0 loops=2)
@@ -1826,14 +1829,14 @@ explain (analyze, costs off, summary off, timing off) execute ab_q6 (1, 0, 0);
                                             QUERY PLAN                                             
 ---------------------------------------------------------------------------------------------------
  Finalize Aggregate (actual rows=1 loops=1)
-   ->  Gather (actual rows=2 loops=1)
+   ->  Gather (actual rows=1 loops=1)
          Workers Planned: 1
-         Workers Launched: 1
-         ->  Partial Aggregate (actual rows=1 loops=2)
-               ->  Nested Loop (actual rows=0 loops=2)
-                     ->  Parallel Seq Scan on lprt_a a (actual rows=51 loops=2)
+         Workers Launched: 0
+         ->  Partial Aggregate (actual rows=1 loops=1)
+               ->  Nested Loop (actual rows=0 loops=1)
+                     ->  Parallel Seq Scan on lprt_a a (actual rows=102 loops=1)
                            Filter: (a = ANY ('{1,0,0}'::integer[]))
-                           Rows Removed by Filter: 1
+                           Rows Removed by Filter: 2
                      ->  Append (actual rows=0 loops=102)
                            ->  Index Scan using ab_a1_b1_a_idx on ab_a1_b1 (actual rows=0 loops=2)
                                  Index Cond: (a = a.a)
@@ -1860,14 +1863,14 @@ explain (analyze, costs off, summary off, timing off) execute ab_q6 (1, 0, 0);
                                          QUERY PLAN                                         
 --------------------------------------------------------------------------------------------
  Finalize Aggregate (actual rows=1 loops=1)
-   ->  Gather (actual rows=2 loops=1)
+   ->  Gather (actual rows=1 loops=1)
          Workers Planned: 1
-         Workers Launched: 1
-         ->  Partial Aggregate (actual rows=1 loops=2)
-               ->  Nested Loop (actual rows=0 loops=2)
-                     ->  Parallel Seq Scan on lprt_a a (actual rows=50 loops=2)
+         Workers Launched: 0
+         ->  Partial Aggregate (actual rows=1 loops=1)
+               ->  Nested Loop (actual rows=0 loops=1)
+                     ->  Parallel Seq Scan on lprt_a a (actual rows=100 loops=1)
                            Filter: (a = ANY ('{1,0,0}'::integer[]))
-                           Rows Removed by Filter: 1
+                           Rows Removed by Filter: 2
                      ->  Append (actual rows=0 loops=100)
                            ->  Index Scan using ab_a1_b1_a_idx on ab_a1_b1 (never executed)
                                  Index Cond: (a = a.a)

src/test/regress/sql/partition_prune.sql

@@ -325,8 +325,13 @@ select avg(a) from ab where a between $1 and $2 and b < 4;
 set parallel_setup_cost = 0;
 set parallel_tuple_cost = 0;
 set min_parallel_table_scan_size = 0;
+
+-- set this so we get a parallel plan
 set max_parallel_workers_per_gather = 2;
 
+-- and zero this so that workers don't destabilize the explain output
+set max_parallel_workers = 0;
+
 -- Execute query 5 times to allow choose_custom_plan
 -- to start considering a generic plan.
 execute ab_q4 (1, 8);