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Re: Adding REPACK [concurrently]

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Antonin Houska <ah@cybertec.at>
Cc: Amit Kapila <amit.kapila16@gmail.com>, Mihail Nikalayeu <mihailnikalayeu@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: 2026-04-09T14:20:18Z
Lists: pgsql-hackers 
Hi,

On 2026-04-09 10:43:14 +0200, Antonin Houska wrote:
> What Andres proposed (AFAIU) should help to avoid this problem because
> REPACK's request for AEL would get in front of the VACUUM's request for SUEL
> in the queue.

Note that that already happens today.

This works today (without the error triggering patch):

S1: REPACK starts
S2: LOCK TABLE / VACUUM / ... starts waiting
S1: REPACK tries to get AEL
S1: REPACK's lock requests get reordered in the wait queue to be before S2 and
    just gets the lock
S1: REPACK finishes
S2: lock acquisition completes.

That's because we do already have this "jumping the wait queue" logic, which I
had forgotten about.


What does *not* work is this:

S1: REPACK starts
S2: BEGIN; SELECT 1 FROM table LIMIT 1;
S2: LOCK TABLE / VACUUM / ... starts waiting
S1: REPACK tries to get AEL
S1: lock is not granted, can't be reordered to be before S2, because S2 holds
    conflicting lock, deadlock detector triggers
S2: lock acquisition completes

But with my proposal to properly teach the deadlock detector about assuming
there's a wait edge for the eventual lock upgrade by S1, the first example
would still work, because the lock upgrade would not be considered a hard
cycle, and the second example would have S2 error out.


> Anti-wraparound (failsafe) VACUUM is a bit different case [1] (i.e. it should
> possibly have higher priority than REPACK), but I think this prioritization
> should be implemented in other way than just letting it get in the way of
> REPACK (at the time REPACK is nearly finished).

Yea, it makes no sense to interrupt the long running repack, given that the
new relation will have much less stuff for vacuum to do.

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