Re: Invalid query generated by postgres_fdw with UNION ALL and ORDER BY

On Thu, Mar 7, 2024 at 4:39 PM David Rowley <dgrowleyml@gmail.com> wrote:

> On Thu, 7 Mar 2024 at 19:09, Michał Kłeczek <michal@kleczek.org> wrote:
> >
> > The following query:
> >
> > SELECT * FROM (
> >   SELECT 2023 AS year, * FROM remote_table_1
> >   UNION ALL
> >   SELECT 2022 AS year, * FROM remote_table_2
> > )
> > ORDER BY year DESC;
> >
> > yields the following remote query:
> >
> > SELECT [columns] FROM remote_table_1 ORDER BY 2023 DESC
> >
> > and subsequently fails remote execution.
> >
> >
> > Not really sure where the problem is - the planner or postgres_fdw.
> > I guess it is postgres_fdw not filtering out ordering keys.
>
> Interesting.  I've attached a self-contained recreator for the casual
> passerby.
>
> I think the fix should go in appendOrderByClause().  It's at that
> point we look for the EquivalenceMember for the relation and can
> easily discover if the em_expr is a Const.  I think we can safely just
> skip doing any ORDER BY <const> stuff and not worry about if the
> literal format of the const will appear as a reference to an ordinal
> column position in the ORDER BY clause.
>

deparseSortGroupClause() calls deparseConst() with showtype = 1.
appendOrderByClause() may want to do something similar for consistency. Or
remove it from deparseSortGroupClause() as well?


>
> Something like the attached patch I think should work.
>
> I wonder if we need a test...
>

Yes.

-- 
Best Wishes,
Ashutosh Bapat