On 2016/06/05 23:01, Andreas Seltenreich wrote:
> Creating some foreign tables via postgres_fdw in the regression db of
> master as of de33af8, sqlsmith triggers the following assertion:
>
> TRAP: FailedAssertion("!(((((const Node*)(var))->type) == T_Var))", File:
> "deparse.c", Line: 1116)
>
> gdb says var is holding a T_PlaceHolderVar instead. In a build without
> assertions, it leads to an error later:
>
> ERROR: cache lookup failed for type 0
>
> Recipe:
>
> --8<---------------cut here---------------start------------->8---
> create extension postgres_fdw;
> create server myself foreign data wrapper postgres_fdw;
> create schema fdw_postgres;
> create user mapping for public server myself options (user :'USER');
> import foreign schema public from server myself into fdw_postgres;
> select subq_0.c0 as c0 from
> (select 31 as c0 from fdw_postgres.a as ref_0
> where 93 >= ref_0.aa) as subq_0
> right join fdw_postgres.rtest_vview5 as ref_1
> on (subq_0.c0 = ref_1.a )
> where 92 = subq_0.c0;
> --8<---------------cut here---------------end--------------->8---
Thanks for the example. It seems that postgres_fdw join-pushdown logic
(within foreign_join_ok()?) should reject a join if any PlaceHolderVars in
its targetlist are required above it. Tried to do that with the attached
patch which trivially fixes the reported assertion failure.
A guess from my reading of the patch's thread [1] is that to support such
a case, join deparse code should be able to construct subqueries which it
currently doesn't support. I may be missing something though.
Thanks,
Amit
[1]
https://www.postgresql.org/message-id/CAFjFpRdHgeNOhM0AB6Gxz1eVx_yOqkYwuKddZeB5vPzfBaeCnQ%40mail.gmail.com
diff --git a/contrib/postgres_fdw/postgres_fdw.c b/contrib/postgres_fdw/postgres_fdw.c
index 4d17272..ec86b9a 100644
--- a/contrib/postgres_fdw/postgres_fdw.c
+++ b/contrib/postgres_fdw/postgres_fdw.c
@@ -4036,6 +4036,20 @@ foreign_join_ok(PlannerInfo *root, RelOptInfo *joinrel, JoinType jointype,
return false;
}
+ /*
+ * Cannot push down if any PlaceHolderVars in its result are needed above
+ * the join.
+ */
+ foreach(lc, root->placeholder_list)
+ {
+ PlaceHolderInfo *phinfo = lfirst(lc);
+ Relids relids = joinrel->relids;
+
+ if (bms_nonempty_difference(phinfo->ph_needed, relids) &&
+ bms_is_subset(phinfo->ph_eval_at, relids))
+ return false;
+ }
+
/* Save the join clauses, for later use. */
fpinfo->joinclauses = joinclauses;
--
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