Re: Fix stale comment in parallel_vacuum_main().

On Tue, Jul 28, 2026 at 11:00 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>
> On Mon, Jul 27, 2026 at 6:00 PM Bharath Rupireddy
> <bharath(dot)rupireddyforpostgres(at)gmail(dot)com> wrote:
> >
> > Hi,
> >
> > While working on the parallel heap vacuum feature I noticed a stale
> > comment in parallel_vacuum_main(). It says a parallel vacuum worker
> > must have only PROC_IN_VACUUM set because parallel index vacuum is not
> > supported for autovacuum. That is no longer true after commit
> > 1ff3180ca01, which enabled autovacuum to use parallel vacuum workers.
>
> Good catch!
>
> > IMHO, the assertion itself is still worth keeping. It ensures the
> > parallel worker carries PROC_IN_VACUUM, so that other backends ignore
> > its xmin when computing the removable-tuple horizon, which is what we
> > want for a worker that is part of a vacuum. The leader, whether it is
> > a backend running a VACUUM command or an autovacuum worker, sets
> > PROC_IN_VACUUM when it starts vacuuming the table, and the worker
> > inherits it when it imports the leader's snapshot (see
> > ProcArrayInstallRestoredXmin). PROC_IS_AUTOVACUUM never reaches the
> > worker, because parallel workers are launched as regular background
> > workers rather than autovacuum workers, so the flag is never set on
> > them, and importing the leader's snapshot doesn't carry it over
> > either. So the worker ends up with exactly PROC_IN_VACUUM even under
> > parallel autovacuum, and the assertion holds.
> >
> > The attached patch rewords the comment to explain this. Since PG19 is
> > still in beta, I would like to backpatch this to 19 where 1ff3180ca01
> > was introduced.
> >
> > Thoughts?
>
> I agree with your analysis. And the proposed comment looks clear to
> me. Will push the patch barring any objections.
>

After reviewing the new comment closely, I think it's better to
mention the PROC_VACUUM_FOR_WRAPAROUND flag too as this is also used
for autovacuums. I've attached the updated patch, please review it.

Regards,

--
Masahiko Sawada
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