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

RhodiumToad · RhodiumToad · commit 8bc6a301b21c · 2018-08-28T15:04:19.000+01:00
Commit aa09cd2 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
@@ -5780,7 +5780,8 @@ 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

Original file line number	Diff line number	Diff line change
`@@ -5780,7 +5780,8 @@ find_em_expr_for_rel(EquivalenceClass ec, RelOptInfo rel)`
`5780`	`5780`	`{`
`5781`	`5781`	`EquivalenceMember *em = lfirst(lc_em);`
`5782`	`5782`
`5783`		`- if (bms_is_subset(em->em_relids, rel->relids))`
	`5783`	`+ if (bms_is_subset(em->em_relids, rel->relids) &&`
	`5784`	`+ !bms_is_empty(em->em_relids))`
`5784`	`5785`	`{`
`5785`	`5786`	`/*`
`5786`	`5787`	`* If there is more than one equivalence member whose Vars are`