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Re: Adding REPACK [concurrently]

Antonin Houska <ah@cybertec.at>

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From: Antonin Houska <ah@cybertec.at>
To: Mihail Nikalayeu <mihailnikalayeu@gmail.com>
Cc: Alvaro Herrera <alvherre@alvh.no-ip.org>, Pg Hackers <pgsql-hackers@lists.postgresql.org>, Robert Treat <rob@xzilla.net>
Date: 2026-01-16T18:18:16Z
Lists: pgsql-hackers 
Mihail Nikalayeu <mihailnikalayeu@gmail.com> wrote:

>Antonin Houska <ah@cybertec.at>:
>> 
>> As the test runs pgbench with --client=30 and the default value of
>> max_worker_processes is 8, I'm not sure this is a leak. I've increased this
>> parameter I couldn't see the error anymore.
> 
> Hm, as far as I remember only single repack may be executed in test (because
> of locking on test itself and also REPACK).

The only problem is that the logical decoding system needs to wait during the
setup for all the running transactions to finish. So if REPACK (CONCURRENTLY)
is already running, the next execution will not start until the first is done.

However, that does not restrict the REPACK decoding workers from starting.

>>  I agree that this is due to the missing MVCC safety feature. I commented that
>>  check in the script for now.
> 
> I don't think so. In case of non-MVCC safety we should see 0 or correct sum. But script failed with 490588...
> But should see 500500 (if I correctly calculated sum of numbers from 1 to 1000)...

I was referring to your statement "It may be 0 because non-MVCC
safe". Regarding the non-zero values, I think I finally understand the issue
and even could reproduce some weird behavior using debugger. Since it also
affects logical replication, I'll provide more details (and hopefully propose
a patch) in a separate thread early next week.

In short, it looks like (hopefully very) rare race condition, such that the
snapshot builder can build the initial snapshot before all the commits have
been recorded in CLOG. When that happens, visibility checks don't work
correctly.

-- 
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