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Re: Adding REPACK [concurrently]

Antonin Houska <ah@cybertec.at>

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From: Antonin Houska <ah@cybertec.at>
To: Robert Treat <rob@xzilla.net>
Cc: Alvaro Herrera <alvherre@alvh.no-ip.org>, Mihail Nikalayeu <mihailnikalayeu@gmail.com>, Srinath Reddy Sadipiralla <srinath2133@gmail.com>, Amit Kapila <amit.kapila16@gmail.com>, Matthias van de Meent <boekewurm+postgres@gmail.com>, Pg Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2026-04-08T07:31:55Z
Lists: pgsql-hackers 
Robert Treat <rob@xzilla.net> wrote:

> On Mon, Apr 6, 2026 at 6:22 PM Alvaro Herrera <alvherre@alvh.no-ip.org> wrote:
> > On 2026-Apr-06, Mihail Nikalayeu wrote:
> <snip>
> >
> > Anyway, here's the three missing parts.  I have not yet edited the
> > deadlock-checker one to protect autovacuum from processing tables under
> > repack.
> >
> 
> I have this lingering bit of paranoia that users could end up in a
> situation with a large / long running repack that goes past failsafe
> age which prevents the simpler fix of failsafe autovacuum from
> running. While the repack finishing would resolve this issue, we can't
> know ahead of time that the repack would finish in time, and
> statistically speaking, failsafe autovacuum should generally run much
> quicker than any repack could. I'm not sure if that means we should
> let failsafe vacuum cancel repacks (that seems a bit extreme), but
> maybe we want to help $operator to think about this decision, except
> if we don't allow autovacuum to wait and we don't allow it to respawn,
> I wonder if the end user will ever realize they are in this position.
> Granted, there doesn't seem like a clean fix for this...

If REPACK is not going to finish in time, I think it makes little difference
whether VACUUM is allowed to wait or not: even if it waits, it will start just
too late. One reason to avoid waiting might be to allow autovacuum to work on
other tables in between.

I agree that the DBA should have some guidance to asses whether REPACK or
(failsafe) VACUUM is the appropriate action. While failsafe VACUUM is clearly
a means to avoid XID wraparound, I tend to consider REPACK primarily a command
to remove table bloat. Or is there a situation where REPACK is better even to
avoid the wraparound?

Technically, the deadlock can be avoided by not running DDLs on the table
while REPACK is running. I'm just thinking if, by mentioning this in the
REPACK documentation, we'd admit that the REPACK (CONCURRENTLY) feature is
actually incomplete. On the other hand, if we don't mention the risk of
deadlock, it's a similar situation to not mentioning it for commands like
ALTER TABLE: if ALTER TABLE performs table rewrite, deadlock can also result
in a significant amount of wasted resources. (Of course, it's not the same if
the purpose of REPACK is considered substitute for failsafe VACUUM, but I'm
not sure about that.)

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

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