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In-Reply-To: 
 <CAD21AoBon929k5bfwX=sT397kyFyHyGgaXGCDOj2+7cDiqZa=w@mail.gmail.com>
From: Daniil Davydov <3danissimo@gmail.com>
Date: Tue, 7 Apr 2026 20:32:32 +0700
Message-ID: 
 <CAJDiXghde6v427BzoNbWeeNRyzkTA6jaZdu=DsrD669+cfsBwg@mail.gmail.com>
Subject: Re: POC: Parallel processing of indexes in autovacuum
To: Masahiko Sawada <sawada.mshk@gmail.com>
Cc: Alexander Korotkov <aekorotkov@gmail.com>,
	SATYANARAYANA NARLAPURAM <satyanarlapuram@gmail.com>,
	Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>,
 Sami Imseih <samimseih@gmail.com>,
	Matheus Alcantara <matheusssilv97@gmail.com>,
 Maxim Orlov <orlovmg@gmail.com>,
	Postgres hackers <pgsql-hackers@lists.postgresql.org>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Archived-At: 
 <https://www.postgresql.org/message-id/CAJDiXghde6v427BzoNbWeeNRyzkTA6jaZdu%3DDsrD669%2BcfsBwg%40mail.gmail.com>
Precedence: bulk

Hi,

On Tue, Apr 7, 2026 at 10:49=E2=80=AFAM Masahiko Sawada <sawada.mshk@gmail.=
com> wrote:
>
> > >
> > > While some autovacuum parameters do override GUCs, those are typicall=
y
> > > local to the process (like cost delay). Parallel workers, however, ar=
e
> > > a shared system-wide resource. In a multi-tenant environment, allowin=
g
> > > a single table's reloption to bypass the
> > > autovacuum_max_parallel_workers =3D 0 limit could lead to unexpected
> > > exhaustion of the worker pool.
> >
> > Will this exhaustion really be unexpected? If we describe such an abili=
ty in
> > the documentation, and the user uses it, then everything is fair. Even =
if
> > administrator forgets that he enabled av_parallel_workers reloption som=
ewhere,
> > then he can :
>
> How can DBAs prevent parallel workers from being exhaustly used if
> users set a high value to the reloption?
>

Only manual control. Since DBA increased reloption manually, it is OK to as=
k
him to manually decrease it.

> > 1)
> > Check the logfile (if log level is not too high) searching for logs lik=
e
> > "parallel workers: index vacuum: N planned, N launched in total".
> > 2)
> > Run a query that selects all tables which have av_parallel_workers > 0.
>
> Does that mean DBAs would need to run these queries periodically?

Not really. I say that even if DBA has lost control on the parallel a/v
workers, it has an ability to find these bottlenecks.

> I don't think that in a multi-tenant environment, DBAs can (or should)
> execute ALTER TABLE on user-owned tables just to fix resource issues.
>

Well, the people I talked to had a different opinion which is based on clie=
nts
feedback : what is acceptable and what is not. I don't think that we can
convince each other, so let it be as it is :)

But if you don't mind continuing to discuss this topic (perhaps with the
involvement of other people), I would love to create a new thread for it.

> > I guess that the question is : "Is it normal if the GUC parameter will =
lose
> > ability to turn off parallel a/v everywhere after the user has manually=
 raised
> > the value for the av_parallel_workers reloption on a few tables?". If t=
he
> > answer is "Yes", I don't see any obstacles for us to allow overriding t=
he GUC
> > parameter via reloption.
>
> I think the answer is no, particularly for this parameter. Since it
> controls a system-wide shared resource, it should be capped by a GUC
> to ensure centralized management, consistent with other
> parallel-query-related GUCs and reloptions.

OK. I believe that "global switch" will also be pretty handy for many use c=
ases.

> Thank you! Pushed.

Great news! Thank you very much for your help, Masahiko-san!

--
Best regards,
Daniil Davydov