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Re: Adding REPACK [concurrently]

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Mihail Nikalayeu <mihailnikalayeu@gmail.com>
Cc: Antonin Houska <ah@cybertec.at>, Amit Kapila <amit.kapila16@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: 2026-04-12T14:05:34Z
Lists: pgsql-hackers 
Hi,

On 2026-04-12 15:31:20 +0200, Mihail Nikalayeu wrote:
> On Thu, Apr 9, 2026 at 4:20 PM Andres Freund <andres@anarazel.de> wrote:
> > 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.
> 
> Attached patch contains some (maybe naive) POC for similar approach.
> It adds a 'deadlock_protected' flag, which changes how the deadlock
> detector cancels backends.
> 
> Instead of cancelling the backend entered the deadlock detector - it
> cancel some another (nearest hard edge) until it is possible to get the
> lock (either by
> reordering or directly).

I don't think that's as good.  The problem is that that way you're only
detecting the deadlocks once they have materialized (i.e. once repack actually
does the lock upgrade), rather than cancelling when we know that the problem
starts.  Having sessions pointlessly blocked for many hours is bad.


> Also, I added test cases with the scenarios you mentioned into the repack
> spec - to ensure repack is still working while other backend are cancelled.

I think we should perhaps commit spec tests for these (I've not yet reviewed
them, but in principle), even before we fix the problem. It's good to document
the current behavior and have a comment that the wrongly cancelled case should
not trigger an error.

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