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Re: Adding REPACK [concurrently]

Antonin Houska <ah@cybertec.at>

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From: Antonin Houska <ah@cybertec.at>
To: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>
Cc: Alvaro Herrera <alvherre@alvh.no-ip.org>, Amit Kapila <amit.kapila16@gmail.com>, "Hayato Kuroda (Fujitsu)" <kuroda.hayato@fujitsu.com>, Srinath Reddy Sadipiralla <srinath2133@gmail.com>, Mihail Nikalayeu <mihailnikalayeu@gmail.com>, Matthias van de Meent <boekewurm+postgres@gmail.com>, Pg Hackers <pgsql-hackers@lists.postgresql.org>, Robert Treat <rob@xzilla.net>
Date: 2026-04-10T13:21:45Z
Lists: pgsql-hackers 
Zhijie Hou (Fujitsu) <houzj.fnst@fujitsu.com> wrote:

> When testing REPACK concurrently, I noticed that all WALs are retained from
> the moment REPACK begins copying data to the new table until the command
> finishes replaying concurrent changes on the new table and stops the repack
> decoding worker.
> 
> I understand the reason: the REPACK command itself starts a long-running
> transaction, and logical decoding does not advance restart_lsn beyond the
> oldest running transaction's start position. As a result, slot.restart_lsn
> remains unchanged, preventing the checkpointer from recycling WALs.

I think you're right, sorry for the omission.

> However, since REPACK can run for a long time (hours or even days), I'd like
> to confirm whether this is expected behavior or if we plan to improve it
> in the future ? And additionally,

Yes, it will be improved. I have a draft patch for it, will rebase and post it
soon. The plan is to:

1) preserve the original xmin/xmax of the tuples when we insert them into the
new heap. Thus, besides achieving MVCC safety, we won't need XID assigned for
most of the time.

2) do catalog changes in separate transactions - XID needed here, but these
transactions take very short time.

3) use a single snapshot only for limited number of tuples/pages. When more
data needs to be copied, a new snapshot is built, supposedly with higher
->xmin than the prevous one.

> IIUC, REPACK without using concurrent option does not have this issue.

It does not have the WAL recycling issue because it does not need to read
WAL. However it also runs in a long transaction. Even though it does not need
XID for the actual heap rewriting, it gets one at the moment it locks the
table using AccessExclusiveLock (which is at the very beginning).

> Given that we do not restart a REPACK, I think the repack decoding worker
> should be able to advance restart_lsn each time after writing changes
> (similar to how a physical slot behaves). To illustrate this, I've written
> a patch (attached) that implements this approach, and it works fine for me.

LGTM, thanks!

> BTW, catalog_xmin also won't advance, but that seems not a big issue as
> the REPACK transaction itself also holds a snapshot that retains catalog tuples,
> so advancing catalog_xmin wouldn't change the situation anyway.

The snapshot "resetting" (mentioned above) should fix this problem too.

-- 
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