Re: Question about InvalidatePossiblyObsoleteSlot()

Hi,

On Thu, Oct 16, 2025 at 11:32:03AM -0700, Masahiko Sawada wrote:
> On Thu, Oct 16, 2025 at 1:23 AM Bertrand Drouvot
> <bertranddrouvot(dot)pg(at)gmail(dot)com> wrote:
> >
> > I'm not sure. The existing logging shows when we invalidate a slot, so the
> > absence of that message indicates we skipped it. Users can also verify the
> > slot status in the view if needed.
> >
>
> I'm somewhat confused. As the commit message of 818fefd8fd4 says, the
> invalidation of an active slot is done in two steps:
>
> - Termination of the backend holding it, if any.
> - Report that the slot is obsolete, with a conflict cause depending on
> the slot's data.
>
> Before commit 818fefd8fd4, it was possible that if slot's
> effective_xmin, catalog_effective_xmin, or restart_lsn gets advanced
> between two steps, we ended up skipping the actual invalidation for
> the slot.

If advancing far enough to reach RS_INVAL_NONE, right.

> What the patch proposed on this thread does is that we allow such pre-
> 818fefd8fd4 behavior only for restart_lsn. That is, we skip the actual
> slot invalidation if the slot's restart_lsn gets advanced and exceeds
> the oldestLSN between two steps.

Exactly.

> I thought the reason why we moved away from pre-818fefd8fd4 was that
> we wanted to avoid reporting only "terminated" and to report both
> things consistently. But if we can accept pre-818fefd8fd4 for
> restart_lsn, why can we not do the same for effective_xmin and
> catalog_effective_xmin?
> I guess that the same is true also for
> effective_xmin and catalog_effective_xmin that we can skip the actual
> slot invalidation if those values get advanced and exceed
> snapshotConflictHorizon.

I think this is safe to do for the restart_lsn because that's the same process
(checkpointer/startup) that is executing InvalidatePossiblyObsoleteSlot()
and will remove the WALs. It can not remove the WALs as long as it is busy in
InvalidatePossiblyObsoleteSlot() so that I think this is safe for the restart_lsn
case.

I'm not that confident for the xmin cases, do you think that's also safe to not
invalidate the slots for effective_xmin and catalog_effective_xmin if they
advance far enough?

Regards,

--
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com