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Re: Adding REPACK [concurrently]

Amit Kapila <amit.kapila16@gmail.com>

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From: Amit Kapila <amit.kapila16@gmail.com>
To: Mihail Nikalayeu <mihailnikalayeu@gmail.com>
Cc: Andres Freund <andres@anarazel.de>, Alvaro Herrera <alvherre@alvh.no-ip.org>, Antonin Houska <ah@cybertec.at>, 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: 2026-04-09T04:54:05Z
Lists: pgsql-hackers 
On Thu, Apr 9, 2026 at 5:08 AM Mihail Nikalayeu
<mihailnikalayeu@gmail.com> wrote:
>
> On Wed, Apr 8, 2026 at 7:22 PM Andres Freund <andres@anarazel.de> wrote:
> > I don't think this is a viable path.  You need to prevent any further lock
> > acquisitions on the relation to be able to swap it, not just conflicting DDL.
>
> AFAIU, Amit's main idea is that we currently upgrade the lock instead
> of **releasing and re-acquiring** it because we fear DDL between those
> actions.
> DML actions are ok for us; REPACK will wait for them while getting
> AEL. Later changes will be applied by REPACK backend while holding
> AEL.
>

Yes, this is exactly what I had in mind.

> One more thing we may prevent from sneaking into that hole is a
> VACUUM. It will not break anything, but will be huge waste of time and
> resources.
>

We can prevent other commands (if required) by checking the
rel_in_use_by_repack flag but I thought for the initial version it is
better to do what is minimally required.

> > And you need to wait for all pre-existing locks to have been released.  That
> > doesn't really get easier by what you propose.
>
>  Do you mean locks from other sessions accessing the table? Is it done
> automatically while waiting for AEL?
>

Right and this is already the case with the code where locks
not-conflicting with ShareUpdateExclusiveLock could be present before
we try to upgrade the lock. The point was we will not let DDL execute
on the table after the new flag (rel_in_use_by_repack) is set and we
released the ShareUpdateExclusiveLock.

> > I don't think CheckTableNotInUse() would work anyway - don't we already hold
> > locks by the point we call it?
>
> Yes, the DDL session already holds locks by the time
> CheckTableNotInUse is called - but is that really the problem? They
> will be released on error.
>

Yes, that is the key to solve this problem. Let me take an example to
explain this a bit more. Right now, the problem can happen in the
following kind of sequence.

Session-1:
REPACK (CONCURRENTLY) foo;

-- now say after the above command has acquired
ShareUpdateExclusiveLock and is doing the work of copying the table,
Session-2 did following actions.

Session-2:
Select * from foo; -- this is allowed
ALTER TABLE foo ADD COLUMN c2; -- this will blocked as Session-1
already has acquired ShareUpdateExclusiveLock

Session-1:
-- continues and tries to upgrade the lock to AEL. This leads to
deadlock ERROR in the current session doing REPACK (CONCURRENTLY).

Now, with the solution I proposed, because we will release
ShareUpdateExclusiveLock after setting a flag like
rel_in_use_by_repack, the ALTER TABLE in session-2 will succeed but
will error_out by CheckTableNotInUse(or a similar function that checks
rel_in_use_by_repack). Both with and without this solution, acquiring
AEL by REPACK (CONCURRENTLY) needs to wait concurrent locks like the
one for SELECT in above example that could have been acquired during
the time REPACKing had ShareUpdateExclusiveLock.

> > And even if that were not the case, there are
> > several paths to locking relations that don't ever go anywhere near
> > CheckTableNotInUse().
>
> But those aren't DDL, so they shouldn't be the problem (CREATE TRIGGER
> might be - it seems to ignore CheckTableNotInUse, but perhaps it's
> fine).
>
> So, in my undeerstanding Amit's idea has two parts: "set flag and
> release/re-acquire" + "use CheckTableNotInUse (or some place like
> that) to check the flag and fail for DDL commands."
>

True, this is the core of the idea.

-- 
With Regards,
Amit Kapila.

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