Flaky test: Force correct plan (#3203)

Failing test: https://app.circleci.com/jobs/github/citusdata/citus/23148

src/test/regress/expected/multi_partitioning.out

@@ -1595,7 +1595,14 @@ SELECT * FROM partitioning_hash_test JOIN partitioning_hash_join_test USING (id,
                                  ->  Parallel Seq Scan on partitioning_hash_test_2_1660032 partitioning_hash_test_2
 (18 rows)
 
--- set partition-wise join on
+-- set partition-wise join on and parallel to off
+SELECT success FROM run_command_on_workers('alter system set max_parallel_workers_per_gather = 0');
+ success 
+---------
+ t
+ t
+(2 rows)
+
 SELECT success FROM run_command_on_workers('alter system set enable_partitionwise_join to on');
  success 
 ---------
@@ -1611,6 +1618,7 @@ SELECT success FROM run_command_on_workers('select pg_reload_conf()');
 (2 rows)
 
 SET enable_partitionwise_join TO on;
+ANALYZE partitioning_hash_test, partitioning_hash_join_test;
 EXPLAIN (COSTS OFF)
 SELECT * FROM partitioning_hash_test JOIN partitioning_hash_join_test USING (id, subid);
                                                                                   QUERY PLAN                                                                                  
@@ -1699,6 +1707,13 @@ SELECT success FROM run_command_on_workers('alter system reset enable_indexonlys
  t
 (2 rows)
 
+SELECT success FROM run_command_on_workers('alter system reset max_parallel_workers_per_gather');
+ success 
+---------
+ t
+ t
+(2 rows)
+
 SELECT success FROM run_command_on_workers('select pg_reload_conf()');
  success 
 ---------

src/test/regress/sql/multi_partitioning.sql

@@ -998,11 +998,13 @@ SELECT success FROM run_command_on_workers('select pg_reload_conf()');
 EXPLAIN (COSTS OFF)
 SELECT * FROM partitioning_hash_test JOIN partitioning_hash_join_test USING (id, subid);
 
--- set partition-wise join on
+-- set partition-wise join on and parallel to off
+SELECT success FROM run_command_on_workers('alter system set max_parallel_workers_per_gather = 0');
 SELECT success FROM run_command_on_workers('alter system set enable_partitionwise_join to on');
 SELECT success FROM run_command_on_workers('select pg_reload_conf()');
 
 SET enable_partitionwise_join TO on;
+ANALYZE partitioning_hash_test, partitioning_hash_join_test;
 
 EXPLAIN (COSTS OFF)
 SELECT * FROM partitioning_hash_test JOIN partitioning_hash_join_test USING (id, subid);
@@ -1019,6 +1021,7 @@ SELECT success FROM run_command_on_workers('alter system reset enable_mergejoin'
 SELECT success FROM run_command_on_workers('alter system reset enable_nestloop');
 SELECT success FROM run_command_on_workers('alter system reset enable_indexscan');
 SELECT success FROM run_command_on_workers('alter system reset enable_indexonlyscan');
+SELECT success FROM run_command_on_workers('alter system reset max_parallel_workers_per_gather');
 SELECT success FROM run_command_on_workers('select pg_reload_conf()');
 
 RESET enable_partitionwise_join;