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Re: Adding REPACK [concurrently]

Antonin Houska <ah@cybertec.at>

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From: Antonin Houska <ah@cybertec.at>
To: Alvaro Herrera <alvherre@alvh.no-ip.org>
Cc: Matthias van de Meent <boekewurm+postgres@gmail.com>, Mihail Nikalayeu <mihailnikalayeu@gmail.com>, Pg Hackers <pgsql-hackers@lists.postgresql.org>, Robert Treat <rob@xzilla.net>
Date: 2026-03-17T10:53:40Z
Lists: pgsql-hackers 
Alvaro Herrera <alvherre@alvh.no-ip.org> wrote:

> On 2026-Mar-16, Matthias van de Meent wrote:
> 
> > On Mon, 16 Mar 2026 at 21:15, Antonin Houska <ah@cybertec.at> wrote:
> 
> > > Anyway (fortunately?), the concurrent use of slots by REPACK is limited
> > > because, during the initialization of logical decoding, the backend needs to
> > > wait for all the transactions having XID assigned to finish, and these include
> > > the already running REPACK commands. See SnapBuildWaitSnapshot() and callers
> > > if you're interested in details.
> > 
> > Huh, so would you be able to run more than one Repack Concurrently in
> > the same database? ISTM that would not be possible, apart from
> > possibly a mechanism comparable to the SAFE_IN_IC flag (to not wait on
> > those backends).
> 
> Yeah, this sounds kind of bad news ...

Admittedly, it is a problem. I tried to address this in pg_squeeze by
pre-allocating slots when it's clear (due to scheduling) that more than one
table needs to be processed. This was an effort to achieve the best possible
performance rather than a response to complaints of users about low
throughput. Nevertheless, I'm glad I happened to mention it before it's too
late.

Regarding solution, a flag like SAFE_IN_IC alone does not help. The
information that particular transaction is used by REPACK (and therefore it
does not have to be decoded) would need to be propagated to the
xl_running_xacts WAL record too.

The enhancements I wrote for PG 20 (not all of them posted yet) that aim at
eliminating the impact of REPACK on VACUUM xmin horizon should fix this
problem: due to the MVCC-safety (i.e. preserving xmin/xmax of the tuples),
REPACK will not need XID assigned (except for catalog changes, which will
happen in separate transactions), so it won't block the logical decoding setup
of other backends.

So the question is whether we should implement a workaround for PG 19, that
won't be needed in v20.

-- 
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