Add a test case for a316a3bc

a316a3bc fixed the code in build_simpl_rel() that propagates
RelOptInfo.userid from parent to child rels so that it works
correctly for the child rels of a UNION ALL subquery rel, though
no tests were added in that commit.  So do so here.

As noted in the discussion, coming up with a test case in the core
regression suite for this fix has turned out to be tricky, so the
test case is added to the postgres_fdw's suite instead.
postgresGetForeignRelSize()'s use of user mapping for the user
specified in RelOptInfo.userid makes it relatively easier to craft
a test case around.

Discussion: https://postgr.es/m/CA%2BHiwqH91GaFNXcXbLAM9L%3DzBwUmSyv699Mtv3i1_xtk9Xec_A%40mail.gmail.com
Backpatch-through: 16

contrib/postgres_fdw/expected/postgres_fdw.out

@@ -2689,6 +2689,48 @@ SELECT t1.c1, t2.c2 FROM v4 t1 LEFT JOIN ft5 t2 ON (t1.c1 = t2.c1) ORDER BY t1.c
 (10 rows)
 
 ALTER VIEW v4 OWNER TO regress_view_owner;
+-- ====================================================================
+-- Check that userid to use when querying the remote table is correctly
+-- propagated into foreign rels present in subqueries under an UNION ALL
+-- ====================================================================
+CREATE ROLE regress_view_owner_another;
+ALTER VIEW v4 OWNER TO regress_view_owner_another;
+GRANT SELECT ON ft4 TO regress_view_owner_another;
+ALTER FOREIGN TABLE ft4 OPTIONS (ADD use_remote_estimate 'true');
+-- The following should query the remote backing table of ft4 as user
+-- regress_view_owner_another, the view owner, though it fails as expected
+-- due to the lack of a user mapping for that user.
+EXPLAIN (VERBOSE, COSTS OFF) SELECT * FROM v4;
+ERROR:  user mapping not found for "regress_view_owner_another"
+-- Likewise, but with the query under an UNION ALL
+EXPLAIN (VERBOSE, COSTS OFF) SELECT * FROM (SELECT * FROM v4 UNION ALL SELECT * FROM v4);
+ERROR:  user mapping not found for "regress_view_owner_another"
+-- Should not get that error once a user mapping is created
+CREATE USER MAPPING FOR regress_view_owner_another SERVER loopback OPTIONS (password_required 'false');
+EXPLAIN (VERBOSE, COSTS OFF) SELECT * FROM v4;
+                    QUERY PLAN                    
+--------------------------------------------------
+ Foreign Scan on public.ft4
+   Output: ft4.c1, ft4.c2, ft4.c3
+   Remote SQL: SELECT c1, c2, c3 FROM "S 1"."T 3"
+(3 rows)
+
+EXPLAIN (VERBOSE, COSTS OFF) SELECT * FROM (SELECT * FROM v4 UNION ALL SELECT * FROM v4);
+                       QUERY PLAN                       
+--------------------------------------------------------
+ Append
+   ->  Foreign Scan on public.ft4
+         Output: ft4.c1, ft4.c2, ft4.c3
+         Remote SQL: SELECT c1, c2, c3 FROM "S 1"."T 3"
+   ->  Foreign Scan on public.ft4 ft4_1
+         Output: ft4_1.c1, ft4_1.c2, ft4_1.c3
+         Remote SQL: SELECT c1, c2, c3 FROM "S 1"."T 3"
+(7 rows)
+
+DROP USER MAPPING FOR regress_view_owner_another SERVER loopback;
+DROP OWNED BY regress_view_owner_another;
+DROP ROLE regress_view_owner_another;
+ALTER FOREIGN TABLE ft4 OPTIONS (SET use_remote_estimate 'false');
 -- cleanup
 DROP OWNED BY regress_view_owner;
 DROP ROLE regress_view_owner;

contrib/postgres_fdw/sql/postgres_fdw.sql

@@ -714,6 +714,29 @@ SELECT t1.c1, t2.c2 FROM v4 t1 LEFT JOIN ft5 t2 ON (t1.c1 = t2.c1) ORDER BY t1.c
 SELECT t1.c1, t2.c2 FROM v4 t1 LEFT JOIN ft5 t2 ON (t1.c1 = t2.c1) ORDER BY t1.c1, t2.c1 OFFSET 10 LIMIT 10;
 ALTER VIEW v4 OWNER TO regress_view_owner;
 
+-- ====================================================================
+-- Check that userid to use when querying the remote table is correctly
+-- propagated into foreign rels present in subqueries under an UNION ALL
+-- ====================================================================
+CREATE ROLE regress_view_owner_another;
+ALTER VIEW v4 OWNER TO regress_view_owner_another;
+GRANT SELECT ON ft4 TO regress_view_owner_another;
+ALTER FOREIGN TABLE ft4 OPTIONS (ADD use_remote_estimate 'true');
+-- The following should query the remote backing table of ft4 as user
+-- regress_view_owner_another, the view owner, though it fails as expected
+-- due to the lack of a user mapping for that user.
+EXPLAIN (VERBOSE, COSTS OFF) SELECT * FROM v4;
+-- Likewise, but with the query under an UNION ALL
+EXPLAIN (VERBOSE, COSTS OFF) SELECT * FROM (SELECT * FROM v4 UNION ALL SELECT * FROM v4);
+-- Should not get that error once a user mapping is created
+CREATE USER MAPPING FOR regress_view_owner_another SERVER loopback OPTIONS (password_required 'false');
+EXPLAIN (VERBOSE, COSTS OFF) SELECT * FROM v4;
+EXPLAIN (VERBOSE, COSTS OFF) SELECT * FROM (SELECT * FROM v4 UNION ALL SELECT * FROM v4);
+DROP USER MAPPING FOR regress_view_owner_another SERVER loopback;
+DROP OWNED BY regress_view_owner_another;
+DROP ROLE regress_view_owner_another;
+ALTER FOREIGN TABLE ft4 OPTIONS (SET use_remote_estimate 'false');
+
 -- cleanup
 DROP OWNED BY regress_view_owner;
 DROP ROLE regress_view_owner;