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Re: Adding REPACK [concurrently]

Antonin Houska <ah@cybertec.at>

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From: Antonin Houska <ah@cybertec.at>
To: Andres Freund <andres@anarazel.de>
Cc: Mihail Nikalayeu <mihailnikalayeu@gmail.com>, 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: 2026-04-15T18:28:24Z
Lists: pgsql-hackers 
Andres Freund <andres@anarazel.de> wrote:

> On 2026-04-15 16:50:11 +0200, Antonin Houska wrote:
> > I thought of a "hypothetical graph", which would include the to-be-granted
> > lock, but the major issue is that it will not work correctly without the
> > locking the LMGR's LW locks we do in CheckDeadLock():
> > 
> >     for (i = 0; i < NUM_LOCK_PARTITIONS; i++)
> >         LWLockAcquire(LockHashPartitionLockByIndex(i), LW_EXCLUSIVE);
> > 
> > And obviously, doing this each time we want to insert a lock into the queue
> > would be bad for performance.
> 
> Hence my suggestion to do this as part of the deadlock check. Then we don't do
> this unnecessary work outside of the case where we actually need it.

> That does need to deal with the case of the deadlock check running first in
> the backend doing repack, but that's not that hard - I think it'd be good
> enough to set its deadlock timeout temporarily to a higher value. The backend
> *should* still run the deadlock detector, because it could probably still get
> into a deadlock (e.g. due to a pg_class access or something).

Yes, the question is when we should run that check. I thought that it should
happen during each lock acquisition, and that made me worried about
performance. AFAIU you suggest REPACK to do something like:


1. Acquire ShareUpdateExclusiveLock

2. Perform the "enhanced check" to see if a future request for
   AccessExclusiveLock can trigger a deadlock.

3. Do major part of the work (copy the table, build indexes, ...)

4. Request AccessExclusive lock.

5. Finish the work (process the remaining concurrent changes and swap the
   table files).


This makes me concerned that if another session does

BEGIN;
TABLE t;

between steps 2 and 4, and something like

-- Get AccessExclusiveLock
ALTER TABLE t ADD COLUMN j int;

just after step 4, the two session will probably up in a deadlock anyway. In
other words, even if REPACK does the check early, it does not prevent other
sessions from getting in the way.

Maybe I'm still missing something.


-- 
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