Refine RLS regression test

Simplify the multi_rls_join_distribution_key regression test by removing the
EXPLAIN query. The EXPLAIN output is not essential for validating the RLS join
fix and can vary across PostgreSQL major versions, potentially
causing test instability.

src/test/regress/expected/multi_rls_join_distribution_key.out

@@ -4,9 +4,6 @@
 -- Test that RLS policies with volatile functions don't prevent
 -- Citus from recognizing joins on distribution columns.
 --
--- This addresses GitHub issue #7969 where RLS policies using
--- current_setting() caused PostgreSQL to split equivalence classes,
--- preventing Citus from detecting proper distribution column joins.
 SET citus.next_shard_id TO 1900000;
 SET citus.shard_replication_factor TO 1;
 -- Create test user if not exists
@@ -63,28 +60,6 @@ BEGIN
 END;
 $$;
 -- This query should work with RLS enabled
--- Before the fix, this would fail with:
--- "complex joins are only supported when all distributed tables are
---  co-located and joined on their distribution columns"
-EXPLAIN (COSTS OFF)
-SELECT c.id, t.id
-FROM table_a AS c
-LEFT OUTER JOIN table_b AS t ON c.tenant_id = t.tenant_id;
-                                                 QUERY PLAN
----------------------------------------------------------------------
- Custom Scan (Citus Adaptive)
-   Task Count: 4
-   Tasks Shown: One of 4
-   ->  Task
-         Node: host=localhost port=xxxxx dbname=regression
-         ->  Nested Loop Left Join
-               ->  Seq Scan on table_a_1900000 c
-                     Filter: (tenant_id = (current_setting('session.current_tenant_id'::text))::uuid)
-               ->  Materialize
-                     ->  Seq Scan on table_b_1900004 t
-                           Filter: (tenant_id = (current_setting('session.current_tenant_id'::text))::uuid)
-(11 rows)
-
 SELECT c.id, t.id
 FROM table_a AS c
 LEFT OUTER JOIN table_b AS t ON c.tenant_id = t.tenant_id
@@ -102,5 +77,4 @@ DROP SCHEMA rls_join_test CASCADE;
 NOTICE:  drop cascades to 2 other objects
 DETAIL:  drop cascades to table table_a
 drop cascades to table table_b
--- Drop user (suppress error if it has dependencies)
 DROP USER IF EXISTS app_user;

src/test/regress/sql/multi_rls_join_distribution_key.sql

@@ -4,9 +4,6 @@
 -- Test that RLS policies with volatile functions don't prevent
 -- Citus from recognizing joins on distribution columns.
 --
--- This addresses GitHub issue #7969 where RLS policies using
--- current_setting() caused PostgreSQL to split equivalence classes,
--- preventing Citus from detecting proper distribution column joins.
 
 SET citus.next_shard_id TO 1900000;
 SET citus.shard_replication_factor TO 1;
@@ -37,11 +34,11 @@ SELECT create_distributed_table('table_b', 'tenant_id', colocate_with => 'table_
 GRANT ALL PRIVILEGES ON ALL TABLES IN SCHEMA rls_join_test TO app_user;
 
 -- Insert test data
-INSERT INTO table_a VALUES 
+INSERT INTO table_a VALUES
     ('0194d116-5dd5-74af-be74-7f5e8468eeb7', 1),
     ('0194d116-5dd5-74af-be74-7f5e8468eeb8', 2);
 
-INSERT INTO table_b VALUES 
+INSERT INTO table_b VALUES
     ('0194d116-5dd5-74af-be74-7f5e8468eeb7', 10),
     ('0194d116-5dd5-74af-be74-7f5e8468eeb8', 20);
 
@@ -59,21 +56,14 @@ SET application_name = '0194d116-5dd5-74af-be74-7f5e8468eeb7';
 
 BEGIN;
 -- Set session.current_tenant_id from application_name
-DO $$ 
-DECLARE 
+DO $$
+DECLARE
 BEGIN
     EXECUTE 'SET LOCAL session.current_tenant_id = ' || quote_literal(current_setting('application_name', true));
-END; 
+END;
 $$;
 
 -- This query should work with RLS enabled
--- Before the fix, this would fail with:
--- "complex joins are only supported when all distributed tables are 
---  co-located and joined on their distribution columns"
-EXPLAIN (COSTS OFF)
-SELECT c.id, t.id
-FROM table_a AS c
-LEFT OUTER JOIN table_b AS t ON c.tenant_id = t.tenant_id;
 
 SELECT c.id, t.id
 FROM table_a AS c
@@ -88,5 +78,4 @@ RESET ROLE;
 -- Cleanup: Drop schema and all objects
 DROP SCHEMA rls_join_test CASCADE;
 
--- Drop user (suppress error if it has dependencies)
 DROP USER IF EXISTS app_user;