Date: Tue, 25 Mar 2025 13:09:07 +0800
From: Julien Rouhaud <rjuju123@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Sami Imseih <samimseih@gmail.com>,
	Michael Paquier <michael@paquier.xyz>,
	Christoph Berg <myon@debian.org>,
	PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>,
	ma lz <ma100@hotmail.com>
Subject: Re: query_id: jumble names of temp tables for better
 pg_stat_statement UX
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Hi,

On Tue, Mar 25, 2025 at 12:32:05AM -0400, Tom Lane wrote:
>
> I'm not opining one way or the other on whether squashing an IN list
> is desirable.  My point is that a GUC is a poor way to make --- or
> really, avoid making --- such decisions.  The reasons we took away
> from previous experiments with semantics-determing GUCs were:
>
> 1. The scope of effect of a GUC is seldom what you want for such
> things.  There are going to be some queries in which you want behavior
> A, and some in which you want behavior B, and in the worst case you
> want different behaviors in different parts of the same query.  It's
> really painful to make that happen.
>
> 2. Tools that are not entitled to set the value of the GUC are forced
> to be prepared to cope with any setting.  That can be anywhere from
> painful to impossible.

Didn't that ship already sailed in pg14 when we allowed generating custom
query_id?

Now:

> For the specific context of controlling how query jumbling happens,
> there's still another problem: the actual statement-merging behavior of
> pg_stat_statements will depend on the totality of settings of the GUC
> across the entire system.  It's not very clear to me what will happen
> if different sessions use different settings, much less if people
> change the setting intra-session; but I bet a lot of people will find
> it surprising.  62d712ecf did no one any favors by marking that GUC
> USERSET rather than something that would prevail system-wide.

One of the requirement for the custom query_id was that you shouldn't be
allowed to change the algorithm dynamically, at least not unless a superuser
agrees to maybe break everything, which is why compute_query_id is marked as
PGC_SUSET.  I think that the same reasoning should apply here and if the GUC is
kept it should be at least at that level.