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Re: Adding REPACK [concurrently]

Alvaro Herrera <alvherre@alvh.no-ip.org>

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From: Alvaro Herrera <alvherre@alvh.no-ip.org>
To: Matthias van de Meent <boekewurm+postgres@gmail.com>
Cc: Antonin Houska <ah@cybertec.at>, Mihail Nikalayeu <mihailnikalayeu@gmail.com>, Pg Hackers <pgsql-hackers@lists.postgresql.org>, Robert Treat <rob@xzilla.net>
Date: 2026-03-16T19:34:12Z
Lists: pgsql-hackers 
On 2026-Mar-16, Matthias van de Meent wrote:

> On Mon, 16 Mar 2026 at 16:10, Alvaro Herrera <alvherre@alvh.no-ip.org> wrote:

> > 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.

True, it's not really free in that sense.  But let's put this aside, as
it's not something I'm considering for pg19 anyway.


> 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.

Well, as I said, these repack slots would be separate from the regular
ones for replication and available only for repack, so they would not
interfere with the count of slots used for replication, so the second
point is moot, isn't it?

-- 
Álvaro Herrera               48°01'N 7°57'E  —  https://www.EnterpriseDB.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