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help / color / mirror / Atom feedFrom: Michael Paquier <[email protected]>
To: Tom Lane <[email protected]>
Cc: jian he <[email protected]>
Cc: PostgreSQL-development <[email protected]>
Cc: Richard Guo <[email protected]>
Subject: Re: JumbleQuery ma treat different GROUP BY expr as the same
Date: Sun, 11 Jan 2026 19:12:46 +0900
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
References: <CACJufxEy2W+tCqC7XuJ94r3ivWsM=onKJp94kRFx3hoARjBeFQ@mail.gmail.com>
<[email protected]>
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:
>
> regression=# SELECT calls, rows, query FROM pg_stat_statements ORDER BY query COLLATE "C";
> calls | rows | query
> -------+------+---------------------------------------------------------------
> 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)
>
> 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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