Re: Fix stale comment in parallel_vacuum_main().

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.

Regards,

--
Masahiko Sawada
Amazon Web Services: https://aws.amazon.com