postgres_fdw: Refuse to send FETCH FIRST WITH TIES to remote servers.

Previously, when considering LIMIT pushdown, postgres_fdw failed to
check whether the query has this clause, which led to pushing false
LIMIT clauses, causing incorrect results.

This clause has been supported since v13, so we need to do a
remote-version check before deciding that it will be safe to push such a
clause, but we do not currently have a way to do the check (without
accessing the remote server); disable pushing such a clause for now.

Oversight in commit 357889eb1.  Back-patch to v13, where that commit
added the support.

Per bug #18467 from Onder Kalaci.

Patch by Japin Li, per a suggestion from Tom Lane, with some changes to
the comments by me.  Review by Onder Kalaci, Alvaro Herrera, and me.

Discussion: https://postgr.es/m/18467-7bb89084ff03a08d%40postgresql.org

1 files changed, 5 insertions, 0 deletions

diff --git a/contrib/postgres_fdw/sql/postgres_fdw.sql b/contrib/postgres_fdw/sql/postgres_fdw.sql
index fdbbb4fe3dd..2e5187a7797 100644
--- a/contrib/postgres_fdw/sql/postgres_fdw.sql
+++ b/contrib/postgres_fdw/sql/postgres_fdw.sql
@@ -407,6 +407,11 @@ EXPLAIN (VERBOSE, COSTS OFF)
   SELECT * FROM ft1 t1 WHERE t1.c1 === t1.c2 order by t1.c2 limit 1;
 SELECT * FROM ft1 t1 WHERE t1.c1 === t1.c2 order by t1.c2 limit 1;
 
+-- Ensure we don't ship FETCH FIRST .. WITH TIES
+EXPLAIN (VERBOSE, COSTS OFF)
+SELECT t1.c2 FROM ft1 t1 WHERE t1.c1 > 960 ORDER BY t1.c2 FETCH FIRST 2 ROWS WITH TIES;
+SELECT t1.c2 FROM ft1 t1 WHERE t1.c1 > 960 ORDER BY t1.c2 FETCH FIRST 2 ROWS WITH TIES;
+
 -- Test CASE pushdown
 EXPLAIN (VERBOSE, COSTS OFF)
 SELECT c1,c2,c3 FROM ft2 WHERE CASE WHEN c1 > 990 THEN c1 END < 1000 ORDER BY c1;