Remove references to optimization PG15 reverted

PG15 introduced an optimization on GROUP BY keys that is now reverted on
RC2.

Relevant PG commit:
Revert "Optimize order of GROUP BY keys".
443df6e2db932a7cd6d85ddfb67e11a43345130d

src/test/regress/expected/cte_inline.out

@@ -463,29 +463,6 @@ DEBUG:  Creating router plan
 (5 rows)
 
 \set VERBOSITY default
--- enable_group_by_reordering is a new GUC introduced in PG15
--- it does some optimization of the order of group by keys which results
--- in a different explain output plan between PG13/14 and PG15
--- Hence we set that GUC to off.
-SHOW server_version \gset
-SELECT substring(:'server_version', '\d+')::int >= 15 AS server_version_ge_15
-\gset
-\if :server_version_ge_15
-SET enable_group_by_reordering TO off;
-\endif
-SELECT DISTINCT 1 FROM run_command_on_workers($$ALTER SYSTEM SET enable_group_by_reordering TO off;$$);
- ?column?
----------------------------------------------------------------------
-        1
-(1 row)
-
-SELECT run_command_on_workers($$SELECT pg_reload_conf()$$);
- run_command_on_workers
----------------------------------------------------------------------
- (localhost,57637,t,t)
- (localhost,57638,t,t)
-(2 rows)
-
 EXPLAIN (COSTS OFF) WITH cte_1 AS NOT MATERIALIZED (SELECT * FROM test_table)
 SELECT
 	count(*)
@@ -524,22 +501,6 @@ DEBUG:  join prunable for intervals [1073741824,2147483647] and [0,1073741823]
                                  ->  Seq Scan on test_table_1960000 test_table_1
 (12 rows)
 
-\if :server_version_ge_15
-RESET enable_group_by_reordering;
-\endif
-SELECT DISTINCT 1 FROM run_command_on_workers($$ALTER SYSTEM RESET enable_group_by_reordering;$$);
- ?column?
----------------------------------------------------------------------
-        1
-(1 row)
-
-SELECT run_command_on_workers($$SELECT pg_reload_conf()$$);
- run_command_on_workers
----------------------------------------------------------------------
- (localhost,57637,t,t)
- (localhost,57638,t,t)
-(2 rows)
-
 -- ctes with volatile functions are not
 -- inlined
 WITH cte_1 AS (SELECT *, random() FROM test_table)

src/test/regress/expected/cte_inline_0.out

@@ -463,29 +463,6 @@ DEBUG:  Creating router plan
 (5 rows)
 
 \set VERBOSITY default
--- enable_group_by_reordering is a new GUC introduced in PG15
--- it does some optimization of the order of group by keys which results
--- in a different explain output plan between PG13/14 and PG15
--- Hence we set that GUC to off.
-SHOW server_version \gset
-SELECT substring(:'server_version', '\d+')::int >= 15 AS server_version_ge_15
-\gset
-\if :server_version_ge_15
-SET enable_group_by_reordering TO off;
-\endif
-SELECT DISTINCT 1 FROM run_command_on_workers($$ALTER SYSTEM SET enable_group_by_reordering TO off;$$);
- ?column?
----------------------------------------------------------------------
-        1
-(1 row)
-
-SELECT run_command_on_workers($$SELECT pg_reload_conf()$$);
- run_command_on_workers
----------------------------------------------------------------------
- (localhost,57637,t,t)
- (localhost,57638,t,t)
-(2 rows)
-
 EXPLAIN (COSTS OFF) WITH cte_1 AS NOT MATERIALIZED (SELECT * FROM test_table)
 SELECT
 	count(*)
@@ -524,22 +501,6 @@ DEBUG:  join prunable for intervals [1073741824,2147483647] and [0,1073741823]
                                  ->  Seq Scan on test_table_1960000 test_table_1
 (12 rows)
 
-\if :server_version_ge_15
-RESET enable_group_by_reordering;
-\endif
-SELECT DISTINCT 1 FROM run_command_on_workers($$ALTER SYSTEM RESET enable_group_by_reordering;$$);
- ?column?
----------------------------------------------------------------------
-        1
-(1 row)
-
-SELECT run_command_on_workers($$SELECT pg_reload_conf()$$);
- run_command_on_workers
----------------------------------------------------------------------
- (localhost,57637,t,t)
- (localhost,57638,t,t)
-(2 rows)
-
 -- ctes with volatile functions are not
 -- inlined
 WITH cte_1 AS (SELECT *, random() FROM test_table)

src/test/regress/expected/multi_explain.out

@@ -636,21 +636,6 @@ Aggregate
                                             ->  Seq Scan on events_1400285 events
                                                   Filter: ((event_type)::text = ANY ('{click,submit,pay}'::text[]))
 -- Union and left join subquery pushdown
--- enable_group_by_reordering is a new GUC introduced in PG15
--- it does some optimization of the order of group by keys which results
--- in a different explain output plan between PG13/14 and PG15
--- Hence we set that GUC to off.
-SHOW server_version \gset
-SELECT substring(:'server_version', '\d+')::int >= 15 AS server_version_ge_15
-\gset
-\if :server_version_ge_15
-SET enable_group_by_reordering TO off;
-\endif
-SELECT DISTINCT 1 FROM run_command_on_workers($$ALTER SYSTEM SET enable_group_by_reordering TO off;$$);
-1
-SELECT run_command_on_workers($$SELECT pg_reload_conf()$$);
-(localhost,57637,t,t)
-(localhost,57638,t,t)
 EXPLAIN (COSTS OFF)
 SELECT
 	avg(array_length(events, 1)) AS event_average,
@@ -873,14 +858,6 @@ Sort
                                                                                 Sort Key: events_2.composite_id
                                                                                 ->  Seq Scan on events_1400285 events_2
                                                                                       Filter: ((composite_id >= '(1,-9223372036854775808)'::user_composite_type) AND (composite_id <= '(1,9223372036854775807)'::user_composite_type) AND ((event_type)::text = 'pay'::text))
-\if :server_version_ge_15
-RESET enable_group_by_reordering;
-\endif
-SELECT DISTINCT 1 FROM run_command_on_workers($$ALTER SYSTEM RESET enable_group_by_reordering;$$);
-1
-SELECT run_command_on_workers($$SELECT pg_reload_conf()$$);
-(localhost,57637,t,t)
-(localhost,57638,t,t)
 -- Lateral join subquery pushdown
 -- set subquery_pushdown due to limit in the query
 SET citus.subquery_pushdown to ON;

src/test/regress/expected/multi_select_distinct.out

@@ -3,13 +3,6 @@
 --
 -- Tests select distinct, and select distinct on features.
 --
-SHOW server_version \gset
-SELECT substring(:'server_version', '\d+')::int >= 15 AS server_version_ge_15;
- server_version_ge_15
----------------------------------------------------------------------
- t
-(1 row)
-
 ANALYZE lineitem_hash_part;
 -- function calls are supported
 SELECT DISTINCT l_orderkey, now() FROM lineitem_hash_part LIMIT 0;
@@ -443,13 +436,12 @@ EXPLAIN (COSTS FALSE)
 		GROUP BY l_suppkey, l_linenumber
 		ORDER BY 1,2
 		LIMIT 10;
-                                                      QUERY PLAN
+                                                       QUERY PLAN
 ---------------------------------------------------------------------
  Limit
-   ->  Sort
-         Sort Key: remote_scan.l_suppkey, ((pg_catalog.sum(remote_scan.avg) / pg_catalog.sum(remote_scan.avg_1)))
-         ->  HashAggregate
-               Group Key: remote_scan.l_suppkey, (pg_catalog.sum(remote_scan.avg) / pg_catalog.sum(remote_scan.avg_1))
+   ->  Unique
+         ->  Sort
+               Sort Key: remote_scan.l_suppkey, ((pg_catalog.sum(remote_scan.avg) / pg_catalog.sum(remote_scan.avg_1)))
                ->  HashAggregate
                      Group Key: remote_scan.l_suppkey, remote_scan.worker_column_4
                      ->  Custom Scan (Citus Adaptive)
@@ -460,7 +452,7 @@ EXPLAIN (COSTS FALSE)
                                  ->  HashAggregate
                                        Group Key: l_suppkey, l_linenumber
                                        ->  Seq Scan on lineitem_hash_part_360041 lineitem_hash_part
-(15 rows)
+(14 rows)
 
 -- check the plan if the hash aggreate is disabled. This explain errors out due
 -- to a bug right now, expectation must be corrected after fixing it.
@@ -595,13 +587,12 @@ EXPLAIN (COSTS FALSE)
 		GROUP BY l_suppkey, l_linenumber
 		ORDER BY 1
 		LIMIT 10;
-                                               QUERY PLAN
+                                                QUERY PLAN
 ---------------------------------------------------------------------
  Limit
-   ->  Sort
-         Sort Key: ((sum(remote_scan.avg) / (pg_catalog.sum(remote_scan.avg_1))::double precision))
-         ->  HashAggregate
-               Group Key: (sum(remote_scan.avg) / (pg_catalog.sum(remote_scan.avg_1))::double precision)
+   ->  Unique
+         ->  Sort
+               Sort Key: ((sum(remote_scan.avg) / (pg_catalog.sum(remote_scan.avg_1))::double precision))
                ->  HashAggregate
                      Group Key: remote_scan.worker_column_3, remote_scan.worker_column_4
                      ->  Custom Scan (Citus Adaptive)
@@ -612,7 +603,7 @@ EXPLAIN (COSTS FALSE)
                                  ->  HashAggregate
                                        Group Key: l_suppkey, l_linenumber
                                        ->  Seq Scan on lineitem_hash_part_360041 lineitem_hash_part
-(15 rows)
+(14 rows)
 
 -- check the plan if the hash aggreate is disabled. This explain errors out due
 -- to a bug right now, expectation must be corrected after fixing it.
@@ -671,13 +662,12 @@ EXPLAIN (COSTS FALSE)
 		GROUP BY l_suppkey, l_linenumber
 		ORDER BY 1
 		LIMIT 10;
-                                                                QUERY PLAN
+                                                                 QUERY PLAN
 ---------------------------------------------------------------------
  Limit
-   ->  Sort
-         Sort Key: (((pg_catalog.sum(remote_scan.dis))::bigint + COALESCE((pg_catalog.sum(remote_scan.dis_1))::bigint, '0'::bigint)))
-         ->  HashAggregate
-               Group Key: ((pg_catalog.sum(remote_scan.dis))::bigint + COALESCE((pg_catalog.sum(remote_scan.dis_1))::bigint, '0'::bigint))
+   ->  Unique
+         ->  Sort
+               Sort Key: (((pg_catalog.sum(remote_scan.dis))::bigint + COALESCE((pg_catalog.sum(remote_scan.dis_1))::bigint, '0'::bigint)))
                ->  HashAggregate
                      Group Key: remote_scan.worker_column_3, remote_scan.worker_column_4
                      ->  Custom Scan (Citus Adaptive)
@@ -688,7 +678,7 @@ EXPLAIN (COSTS FALSE)
                                  ->  HashAggregate
                                        Group Key: l_suppkey, l_linenumber
                                        ->  Seq Scan on lineitem_hash_part_360041 lineitem_hash_part
-(15 rows)
+(14 rows)
 
 -- check the plan if the hash aggreate is disabled. This explain errors out due
 -- to a bug right now, expectation must be corrected after fixing it.

src/test/regress/sql/cte_inline.sql

@@ -246,19 +246,6 @@ $Q$);
 
 \set VERBOSITY default
 
--- enable_group_by_reordering is a new GUC introduced in PG15
--- it does some optimization of the order of group by keys which results
--- in a different explain output plan between PG13/14 and PG15
--- Hence we set that GUC to off.
-SHOW server_version \gset
-SELECT substring(:'server_version', '\d+')::int >= 15 AS server_version_ge_15
-\gset
-\if :server_version_ge_15
-SET enable_group_by_reordering TO off;
-\endif
-SELECT DISTINCT 1 FROM run_command_on_workers($$ALTER SYSTEM SET enable_group_by_reordering TO off;$$);
-SELECT run_command_on_workers($$SELECT pg_reload_conf()$$);
-
 EXPLAIN (COSTS OFF) WITH cte_1 AS NOT MATERIALIZED (SELECT * FROM test_table)
 SELECT
 	count(*)
@@ -268,13 +255,6 @@ FROM
 	cte_1 as second_entry
 		USING (key);
 
-\if :server_version_ge_15
-RESET enable_group_by_reordering;
-\endif
-SELECT DISTINCT 1 FROM run_command_on_workers($$ALTER SYSTEM RESET enable_group_by_reordering;$$);
-SELECT run_command_on_workers($$SELECT pg_reload_conf()$$);
-
-
 -- ctes with volatile functions are not
 -- inlined
 WITH cte_1 AS (SELECT *, random() FROM test_table)

src/test/regress/sql/multi_explain.sql

@@ -254,20 +254,6 @@ FROM
 		user_id) AS subquery;
 
 -- Union and left join subquery pushdown
-
--- enable_group_by_reordering is a new GUC introduced in PG15
--- it does some optimization of the order of group by keys which results
--- in a different explain output plan between PG13/14 and PG15
--- Hence we set that GUC to off.
-SHOW server_version \gset
-SELECT substring(:'server_version', '\d+')::int >= 15 AS server_version_ge_15
-\gset
-\if :server_version_ge_15
-SET enable_group_by_reordering TO off;
-\endif
-SELECT DISTINCT 1 FROM run_command_on_workers($$ALTER SYSTEM SET enable_group_by_reordering TO off;$$);
-SELECT run_command_on_workers($$SELECT pg_reload_conf()$$);
-
 EXPLAIN (COSTS OFF)
 SELECT
 	avg(array_length(events, 1)) AS event_average,
@@ -403,12 +389,6 @@ GROUP BY
 ORDER BY
 	count_pay;
 
-\if :server_version_ge_15
-RESET enable_group_by_reordering;
-\endif
-SELECT DISTINCT 1 FROM run_command_on_workers($$ALTER SYSTEM RESET enable_group_by_reordering;$$);
-SELECT run_command_on_workers($$SELECT pg_reload_conf()$$);
-
 -- Lateral join subquery pushdown
 -- set subquery_pushdown due to limit in the query
 SET citus.subquery_pushdown to ON;

src/test/regress/sql/multi_select_distinct.sql

@@ -3,9 +3,6 @@
 --
 -- Tests select distinct, and select distinct on features.
 --
-SHOW server_version \gset
-SELECT substring(:'server_version', '\d+')::int >= 15 AS server_version_ge_15;
-
 ANALYZE lineitem_hash_part;
 
 -- function calls are supported