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.
