Re: Adding REPACK [concurrently]

Robert Treat <rob(at)xzilla(dot)net> wrote:

> On Thu, Apr 9, 2026 at 10:20 AM Andres Freund <andres(at)anarazel(dot)de> wrote:
> > On 2026-04-09 10:43:14 +0200, Antonin Houska wrote:
> > > Anti-wraparound (failsafe) VACUUM is a bit different case [1] (i.e. it should
> > > possibly have higher priority than REPACK), but I think this prioritization
> > > should be implemented in other way than just letting it get in the way of
> > > REPACK (at the time REPACK is nearly finished).
> >
> > Yea, it makes no sense to interrupt the long running repack, given that the
> > new relation will have much less stuff for vacuum to do.
> >
>
> We might be talking about 2 different scenarios. In the case where we
> are at the point of lock escalation, you would probably want the
> repack to get priority over a waiting vacuum, even a failsafe vacuum.
> But outside of that scenario, we can't know that the repack is the
> better option (and statistically it probably isn't) since a repack
> that is actively copying rows might still need to rebuild some large
> number of indexes (or just some really expensive index) which could
> take significantly longer than a failsafe vacuum would need to ensure
> wraparound avoidance. I don't think we'd go as far as saying the
> failsafe vacuum should cancel the repack, but I think ideally we'd
> like it to not be canceled either, since that would increase
> likelihood for dba/monitoring to pick up on the situation, and in the
> case that REPACK fails for some reason, the failsafe vacuum could
> immediately start working without having to go through any additional
> hoops.

What about just teaching the anti-wraparound VACUUM to print out a WARNING if
it could not lock the table in "reasonable" time? The DBA would then have to
consider if the blocking command should be cancelled, whether it's REPACK or
something else.

--
Antonin Houska
Web: https://www.cybertec-postgresql.com