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From: Richard Guo <guofenglinux@gmail.com>
Date: Fri, 10 Apr 2026 23:38:50 +0900
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 <CAMbWs49Wi81GGO_drC4PJgQUehz4_+ekmxHyO9nvLzP1-mJuCw@mail.gmail.com>
Subject: Re: pg17: XX000: no relation entry for relid 0
To: Tender Wang <tndrwang@gmail.com>
Cc: Justin Pryzby <pryzby@telsasoft.com>, pgsql-hackers@lists.postgresql.org,
	Paul Jungwirth <pj@illuminatedcomputing.com>, Tom Lane <tgl@sss.pgh.pa.us>
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On Fri, Apr 10, 2026 at 9:49=E2=80=AFPM Tender Wang <tndrwang@gmail.com> wr=
ote:
> Justin Pryzby <pryzby@telsasoft.com> =E4=BA=8E2026=E5=B9=B44=E6=9C=8810=
=E6=97=A5=E5=91=A8=E4=BA=94 18:54=E5=86=99=E9=81=93=EF=BC=9A
> > CREATE VIEW x AS SELECT NULL::int[]
> > UNION ALL SELECT NULL::int[]
> > UNION ALL SELECT ARRAY[NULL::bigint];
> >
> > SELECT FROM x;
> > ERROR:  XX000: no relation entry for relid 0

Nice catch.  It seems that we need at least three branches to
reproduce this, so that there's a nested UNION ALL whose output type
doesn't match the parent's expected type.  At the outer branch maybe
we can use NULL::bigint[] instead to be a little simpler.

SELECT NULL::int[] UNION ALL SELECT NULL::int[] UNION ALL SELECT NULL::bigi=
nt[];
ERROR:  no relation entry for relid 0

> I didn't think too much at now, a quick fix as below:
> diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/sel=
funcs.c
> index 4160d2d6e24..ff93fc3ac8a 100644
> --- a/src/backend/utils/adt/selfuncs.c
> +++ b/src/backend/utils/adt/selfuncs.c
> @@ -2265,6 +2265,9 @@ estimate_array_length(PlannerInfo *root, Node *arra=
yexpr)
>                 AttStatsSlot sslot;
>                 double          nelem =3D 0;
>
> +               if (IsA(arrayexpr, Var) && ((Var *) arrayexpr)->varno =3D=
=3D 0)
> +                       return 10;
> +
>                 examine_variable(root, arrayexpr, 0, &vardata);
>                 if (HeapTupleIsValid(vardata.statsTuple))
>                 {
>
> Any thoughts?

This looks like the right fix to me.  We can use some comment here.

- Richard