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Re: Adding REPACK [concurrently]

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Mihail Nikalayeu <mihailnikalayeu@gmail.com>
Cc: Antonin Houska <ah@cybertec.at>, Amit Kapila <amit.kapila16@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>, Robert Treat <rob@xzilla.net>
Date: 2026-04-09T14:34:52Z
Lists: pgsql-hackers 
Hi,

On 2026-04-09 10:26:22 -0400, Andres Freund wrote:
> On 2026-04-09 16:06:17 +0200, Mihail Nikalayeu wrote:
> > On Thu, Apr 9, 2026 at 11:26 AM Antonin Houska <ah@cybertec.at> wrote:
> > > Sure, it's possible, but IMO the principal question is whether REPACK should
> > > let VACUUM and DDLs error out, or just let them wait.
> > 
> > One more idea: instead of ERROR in CheckTableNotInUse in case of
> > in_repack - just release the lock and retry a little later (by that
> > time, the repack operation will likely have acquired the AEL).
> 
> I continue to think this approach has no chance of working.  Even if it
> theoretically could, there are lots of paths to locking relations that do not
> go through CheckTableNotInUse(), and we're not going to just route them all
> through CheckTableNotInUse().
> 
> What CheckTableNotInUse() is for is to prevent DDL from changing the structure
> of the table when it is still being referred to.  You can't just call
> CheckTableNotInUse() from a LOCK TABLE - it would trigger wrong errors *ALL
> THE TIME* because a LOCK TABLE does not need to error out just because there's
> also a cursor on the table.
> 
> 
> And it'd trigger lots of bogus errors. See the first example in
> https://postgr.es/m/fpr4nsmyy3mpfrm2mijspr44dgol2cjeke5tyznb4btsznxsgx%40iifdbfe2wl63
> 
> S2 would get the lock and then error out due to the proposed check. Even
> though there's no need for it.

Before you protest that you could just let the non-DDL operation have its
lock, sure, but that's an utterly terrible idea, because it will lead to more
and more work accumulating that then has to happen when the AEL is actually
held.

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