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Re: Adding REPACK [concurrently]

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Robert Treat <rob@xzilla.net>
Cc: Antonin Houska <ah@cybertec.at>, Amit Kapila <amit.kapila16@gmail.com>, Mihail Nikalayeu <mihailnikalayeu@gmail.com>, Alvaro Herrera <alvherre@alvh.no-ip.org>, Srinath Reddy Sadipiralla <srinath2133@gmail.com>, Matthias van de Meent <boekewurm+postgres@gmail.com>, Pg Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2026-04-12T14:02:59Z
Lists: pgsql-hackers 
Hi,

On 2026-04-10 12:14:59 -0400, Robert Treat wrote:
> On Thu, Apr 9, 2026 at 10:20 AM Andres Freund <andres@anarazel.de> 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 really understand why you consider REPACK CONCURRENTLY to be so
different from other existing long-running operations?


> 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

I don't think what I suggested could lead to (auto-)vacuum getting cancelled?
Antonin's earlier patch could, but as long as you only do cancellations if
there's an actual deadlock cycle, VACUUM should not get cancelled, since it
won't already hold a lower level lock?


> , 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.

Not sure I buy this (leaving aside the premise doesn't hold, I think). If you
have monitoring for either long running tasks or tables not getting
autovacuumed, it'd pick up on that regardless of whether autovacuum is getting
cancelled or not.  What realistic monitoring would pick up on autovacuum just
being stuck waiting for a lock for long, but would not pick up on repack
running forever or age(datfrozenxid) getting a bit long in the tooth?

Greetings,

Andres Freund

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Allow old WAL recycling during REPACK CONCURRENTLY

  2. Advance restart_lsn more eagerly in LogicalConfirmReceivedLocation

  3. Remove unnecessary signal handler change

  4. Improve REPACK (CONCURRENTLY) error messages some more

  5. Revert "Allow logical replication snapshots to be database-specific"

  6. Move REPACK (CONCURRENTLY) test out of stock regression tests

  7. REPACK: do not require REPLICATION or LOGIN

  8. Add missing initialization

  9. Simplify declaration of memcpy target

  10. Reserve replication slots specifically for REPACK

  11. doc: Add an example of REPACK (CONCURRENTLY)

  12. Allow logical replication snapshots to be database-specific

  13. Avoid different-size pointer-to-integer cast

  14. Fix valgrind failure

  15. Add CONCURRENTLY option to REPACK

  16. Rename cluster.c to repack.c (and corresponding .h)

  17. Allow index_create to suppress index_build progress reporting

  18. Make index_concurrently_create_copy more general

  19. Document the 'command' column of pg_stat_progress_repack

  20. Introduce the REPACK command

  21. Toggle logical decoding dynamically based on logical slot presence.

  22. Split vacuumdb to create vacuuming.c/h

  23. Remove ReorderBufferTupleBuf structure.

  24. Revert changes to CONCURRENTLY that "sped up" Xmin advance

  25. VACUUM: ignore indexing operations with CONCURRENTLY