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Re: Adding REPACK [concurrently]

Matthias van de Meent <boekewurm+postgres@gmail.com>

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From: Matthias van de Meent <boekewurm+postgres@gmail.com>
To: Alvaro Herrera <alvherre@alvh.no-ip.org>
Cc: Antonin Houska <ah@cybertec.at>, Mihail Nikalayeu <mihailnikalayeu@gmail.com>, Pg Hackers <pgsql-hackers@lists.postgresql.org>, Robert Treat <rob@xzilla.net>
Date: 2026-03-16T19:24:45Z
Lists: pgsql-hackers 
On Mon, 16 Mar 2026 at 16:10, Alvaro Herrera <alvherre@alvh.no-ip.org> wrote:
>
> On 2026-Mar-16, Matthias van de Meent wrote:
>
> > Note that most of my argument hinges on the impact on other, unrelated
> > databases/tables/sessions. Replication slots have a hard cap defined
> > at startup, and effective_wal_level increases the WAL generated by
> > practically all backends.
>
> I'd rather have a new GUC to declare a bunch of additional slots that
> are reserved exclusively for repack, set its default to something like
> 3, and call it a day.  If all repack slots are in use, you don't get to
> run repack, period.
>
> A slot costs nothing if unused, and we really don't want to make the
> interaction with regular replication more complicated than it is today.

There are a few overheads even for unused slots: each slot uses some
shared memory, and many (most) functions that operate on the shared
slot state have O(n_slots) overhead through e.g.
ReplicationSlotsComputeRequiredXmin(); so having a 10k slot limit
whilst using only 1 will be slower for that one active slot than if
the limit was just 10. Granted, that's not a lot of overhead, but it's
not free.

> > However, we don't live in that world, so I am opposed to allowing
> > table owners without REPLICATION to take any/all replication slots.
>
> I think requiring REPACK users to have the REPLICATION priv is rather
> user unfriendly.  Some potential REPACK users might not have any other
> use for replication at all.

I agree it's not user-friendly, but that's the point of limiting
permissions. Users can't install c-functions without SUPERUSER,
because it can cause cluster instability and crashes. Users can't
create slots without REPLICATION, because they'll be able to
negatively impact the whole cluster's performance, and possibly,
stability, when taking up replication slots that otherwise would be
used for critical HA purposes.

Kind regards,

Matthias van de Meent
Databricks (https://www.databricks.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