Feedback-ID: i0fe9450f:Fastmail
Date: Sun, 11 Jan 2026 19:12:46 +0900
From: Michael Paquier <michael@paquier.xyz>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: jian he <jian.universality@gmail.com>,
	PostgreSQL-development <pgsql-hackers@postgresql.org>,
	Richard Guo <guofenglinux@gmail.com>
Subject: Re: JumbleQuery ma treat different GROUP BY expr as the same
Message-ID: <aWN3nkHOSu392MFX@paquier.xyz>
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On Sat, Jan 10, 2026 at 11:46:27AM -0500, Tom Lane wrote:
> It is not what happened before we invented RTE_GROUP.  I tried your
> experiment in v14 and got:
>=20
> regression=3D# SELECT calls, rows, query FROM pg_stat_statements ORDER BY=
 query COLLATE "C";
>  calls | rows |                             query                        =
    =20
> -------+------+----------------------------------------------------------=
-----
>      1 |    1 | SELECT pg_stat_statements_reset() IS NOT NULL AS t
>      1 |    0 | explain(costs off, verbose) select count(*) from t group =
by a
>      1 |    0 | explain(costs off, verbose) select count(*) from t group =
by b
>      1 |    0 | explain(costs off, verbose) select count(*) from t group =
by c
> (4 rows)
>=20
> So I'm inclined to think this was an unintentional change of behavior.

The difference of behavior is between v17 and v18, as an effect of
247dea89f761.  I know that we should not break query ID computations
in stable branches, but v18 is very recent and we still have many
years to support it..  So I would like to suggest that we make an
exception and backpatch a fix to v18.  This behavior is not cool for
users.

This issue also points to a gap in the regression of
pg_stat_statements, where we have never bothered testing patterns of
GROUP BY with the same table and different attributes.  Jian, would
you like to write a patch?
--
Michael

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