Re: postgres_fdw could deparse ArrayCoerceExpr

On Mon, Jul 13, 2026 at 9:47 AM Alexander Pyhalov
<a(dot)pyhalov(at)postgrespro(dot)ru> wrote:
> Alexander Korotkov писал(а) 2026-07-12 19:48:
> > On Sat, Jul 11, 2026 at 5:42 AM Noah Misch <noah(at)leadboat(dot)com> wrote:
> >> On Wed, Jul 16, 2025 at 12:55:44AM +0300, Alexander Korotkov wrote:
> >> > On Wed, Jun 4, 2025 at 11:52 PM Alexander Korotkov <aekorotkov(at)gmail(dot)com> wrote:
> >> > > On Wed, Jun 4, 2025 at 6:15 PM Alexander Pyhalov
> >> > > <a(dot)pyhalov(at)postgrespro(dot)ru> wrote:
> >> > > > Alexander Korotkov писал(а) 2025-06-04 14:29:
> >> > > > > On Wed, Jan 29, 2025 at 11:59 AM Maxim Orlov <orlovmg(at)gmail(dot)com> wrote:
> >> > > > >> One important note here. This patch will change cast behaviour in case
> >> > > > >> of local and foreign types are mismatched.
> >> > > > >> The problem is if we cannot convert types locally, this does not mean
> >> > > > >> that it is also true for a foreign wrapped data.
> >> > > > >> In any case, it's up to the committer to decide whether this change is
> >> > > > >> needed or not.
> >> > > > >
> >> > > > > I have two question regarding this aspect.
> >> > > > > 1) Is it the same with regular type conversion?
> >> > > >
> >> > > > Yes, it's the same.
> >> > > >
> >> > > > CREATE TYPE enum_of_int_like AS enum('1', '2', '3', '4');
> >> > > > CREATE TABLE conversions(id int, d enum_of_int_like);
> >> > > > CREATE FOREIGN TABLE ft_conversions (id int, d char(1))
> >> > > > SERVER loopback options (table_name 'conversions');
> >> > > > SET plan_cache_mode = force_generic_plan;
> >> > > > PREPARE s(varchar) AS SELECT count(*) FROM ft_conversions where d=$1;
> >> > > > EXPLAIN (VERBOSE, COSTS OFF)
> >> > > > EXECUTE s('1');
> >> > > > QUERY PLAN
> >> > > > -------------------------------------------------------------------------------------------
> >> > > > Foreign Scan
> >> > > > Output: (count(*))
> >> > > > Relations: Aggregate on (public.ft_conversions)
> >> > > > Remote SQL: SELECT count(*) FROM public.conversions WHERE ((d =
> >> > > > $1::character varying))
> >> > > > (4 rows)
> >> > > >
> >> > > > EXECUTE s('1');
> >> > > > ERROR: operator does not exist: public.enum_of_int_like = character
> >> > > > varying
> >> > > > HINT: No operator matches the given name and argument types. You might
> >> > > > need to add explicit type casts.
> >>
> >> > > Got it, thank you for the explanation. I thin it's fair that array
> >> > > coercion works the same way as a regular cast.
> >>
> >> I agree with that principle. While the above example shows regular
> >> and array
> >> casts aligned, Fable 5 found the attached test cases where that
> >> alignment is
> >> absent, yielding array-specific wrong query result scenarios. This
> >> thread's
> >> commit 62c3b4c introduced those. I think the test patch's
> >> "implicit-format
> >> ArrayCoerceExpr" is not meaningfully a regression, because scalars
> >> have the
> >> same problem. The other two, "pushed down although the element
> >> conversion
> >> calls a cast" and "CoerceViaIO are pushed down", are array-specific
> >> regressions. Scalars don't get corresponding trouble for those two.
> >> I'm also
> >> attaching Fable 5's report.
> >>
> >> > I've written a commit message for this patch. I'm going to push this
> >> > if no objections.
> >
> > Thank you for catching this. The fix is attached. It ships
> > ArrayCoerceExpr only when its element expression is RelabelType or
> > CaseTestExpr. I propose to backpatch it to pg 19. And we can
> > consider shipping ArrayCoerceExpr with shippable element cast
> > functions for pg 20.
> >
>
> Thanks for report. The suggested fix looks good to me.

Thank you. I'm going to push this if no objections.

------
Regards,
Alexander Korotkov
Supabase