postgres_fdw: don't push ORDER BY with no vars (bug #15352)

Commit aa09cd242 changed a condition in find_em_expr_for_rel from
being a bms_equal comparison of relids to bms_is_subset, in order to
support order by clauses on foreign joins. But this also allows
through the degenerate case of expressions with no Vars at all (and
hence empty relids), including integer constants which will be parsed
unexpectedly on the remote (viz. "ERROR: ORDER BY position 0 is not in
select list" as in the bug report).

Repair by adding an additional !bms_is_empty test.

Backpatch through to 9.6 where the aforementioned change was made.

Per bug #15352 from Maksym Boguk; analysis and patch by me.

Discussion: https://postgr.es/m/153518420278.1478.14875560810251994661@wrigleys.postgresql.org

diff --git a/contrib/postgres_fdw/postgres_fdw.c b/contrib/postgres_fdw/postgres_fdw.c
index d8ae029faa9af0e8f8e3b39918b57a922d268841..5bc8bca45f8d9b709fa3a66387580036db857406 100644 (file)
--- a/contrib/postgres_fdw/postgres_fdw.c
+++ b/contrib/postgres_fdw/postgres_fdw.c
@@ -5169,7 +5169,8 @@ find_em_expr_for_rel(EquivalenceClass *ec, RelOptInfo *rel)
    {
        EquivalenceMember *em = lfirst(lc_em);
 
-       if (bms_is_subset(em->em_relids, rel->relids))
+       if (bms_is_subset(em->em_relids, rel->relids) &&
+           !bms_is_empty(em->em_relids))
        {
            /*
             * If there is more than one equivalence member whose Vars are