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From: Robert Haas <robertmhaas@gmail.com>
Date: Fri, 31 Oct 2025 08:51:34 -0400
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Subject: Re: pg_plan_advice
To: Jakub Wartak <jakub.wartak@enterprisedb.com>
Cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
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On Fri, Oct 31, 2025 at 5:59=E2=80=AFAM Jakub Wartak
<jakub.wartak@enterprisedb.com> wrote:
> > First, any form of user control over the planner tends to be a lightnin=
g rod for criticism around here.
>
> I do not know where this is coming from, but everybody I've talked to
> was saying this is needed to handle real enterprise databases and
> applications. I just really love it, how one could precisely adjust
> the plan with this even with the presence of heavy aliasing:

Thanks for the kind words.

I'll respond to the points about compiler warnings later.

> To attract a little attention to the $thread, the only bigger design
> (usability) question that keeps ringing in my head is how we are going
> to bind it to specific queries without even issuing any SETs(or ALTER
> USER) in the far future in the grand scheme of things. The discussed
> query id (hash), full query text comparison, maybe even strstr(query ,
> "partial hit") or regex all seem to be kind too limited in terms of
> what crazy ORMs can come up with (each query will be potentially
> slightly different, but if optimizer reference points are stable that
> should nail it good enough, but just enabling it for the very specific
> set of queries and not the others [with same aliases] is some major
> challenge).

Yeah, I haven't really dealt with this problem yet.

> Due to this, at some point I was even thinking about some hashes for
> every plan node (including hashes of subplans),
[...]
>
> and then having a way to use `somehashval3` (let's say it's SHA1) as a
> way to activate the necessary advice. Something like having a way to

This doesn't make sense to me, because it seems circular. We can't use
anything in the plan to choose which advice string to use, because the
purpose of the advice string is to influence the choice of plan. In
other words, our choice of what advice string to use has to be based
on the properties of the query, not the plan. We can implement
anything we want to do in terms of exactly how that works: we can use
the query ID, or the query text, or the query node tree.
Hypothetically, we could call out to a user-defined function and pass
the query text or the query node tree as an argument and let it do
whatever it wants to decide on an advice string. The practical problem
here is computational cost -- any computation that gets performed for
every single query is going to have to be pretty cheap to avoid
creating a performance problem. That's why I thought matching on query
ID or exact matching on query text would likely be the most practical
approaches, aside from the obvious alternative of setting and
resetting pg_plan_advice.advice manually. But I haven't really
explored this area too much yet, because I need to get all the basics
working first.

--=20
Robert Haas
EDB: http://www.enterprisedb.com