From: Tom Lane <tgl@sss.pgh.pa.us>
To: exclusion@gmail.com
cc: pgsql-bugs@lists.postgresql.org
Subject: Re: BUG #18652: Planner can not find pathkey item to sort for query
 with expression and expression index
In-reply-to: <18652-deaa782ebcca85d1@postgresql.org>
References: <18652-deaa782ebcca85d1@postgresql.org>
Comments: In-reply-to PG Bug reporting form <noreply@postgresql.org>
	message dated "Wed, 09 Oct 2024 21:00:01 -0000"
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Date: Wed, 09 Oct 2024 17:43:03 -0400
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PG Bug reporting form <noreply@postgresql.org> writes:
> The following query:
> CREATE TABLE t(i int, j int);
> CREATE INDEX idx on t((i + 0));

> SELECT * FROM t,
>   (SELECT i + 0 AS i FROM
>     (SELECT i FROM t UNION ALL SELECT i + 1 FROM t) AS t1
>   ) AS t2
> WHERE t2.i = t.j;

> fails with:
> ERROR:  XX000: could not find pathkey item to sort
> LOCATION:  prepare_sort_from_pathkeys, createplan.c:6350

> The error occurs only when the expression in SELECT i + 0 AS i FROM
> matches the expression in an index.

> Reproduced on REL_10_STABLE .. master.

Hm, this seems quite an old bug: I can reproduce it on 9.1 but
not 9.0.  Unfortunately, that's too far back to build easily
on modern platforms, so I'm not seeing a way to "git bisect"
for more insight.

It looks like we are generating a Path tree in which one of the
inputs to a MergeAppend is a plain unsorted seqscan, which'd
be all right except it doesn't expose the required sort value
in its targetlist.  More later.

			regards, tom lane