Re: BUG #17978: Unexpected error: "wrong varnullingrels (b) (expected (b 5)) for Var 6/2" triggered by JOIN

On Sun, Jun 18, 2023 at 3:57 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:

> Temporarily disabling the setrefs.c crosscheck shows that it's
> trying to create this plan:
>
> which is the wrong join order: the filter condition can't be
> applied at that join level.  So the nullingrel cross-check
> has caught a real bug, but why the bug?


FWIW, I found that we have an existing test query in sql/join.sql that
almost exposes this issue.

explain (costs off)
select * from int4_tbl t1
  left join ((select t2.f1 from int4_tbl t2
                left join int4_tbl t3 on t2.f1 > 0
                where t3.f1 is null) s
             left join tenk1 t4 on s.f1 > 1)
    on s.f1 = t1.f1;

If we change the WHERE clause to 't2.f1 != coalesce(t3.f1, 1)', we will
see this issue.

explain (costs off)
select * from int4_tbl t1
  left join ((select t2.f1 from int4_tbl t2
                left join int4_tbl t3 on t2.f1 > 0
                where t2.f1 != coalesce(t3.f1, 1)) s
             left join tenk1 t4 on s.f1 > 1)
    on s.f1 = t1.f1;
ERROR:  wrong varnullingrels (b) (expected (b 5)) for Var 6/1

Thanks
Richard