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RE: Adding REPACK [concurrently]

Zhijie Hou (Fujitsu) <houzj.fnst@fujitsu.com>

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From: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>
To: Alvaro Herrera <alvherre@alvh.no-ip.org>, Amit Kapila <amit.kapila16@gmail.com>
Cc: "Hayato Kuroda (Fujitsu)" <kuroda.hayato@fujitsu.com>, Antonin Houska <ah@cybertec.at>, 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: 2026-04-10T10:53:53Z
Lists: pgsql-hackers

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Hi,

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.

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

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.

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.

Thoughts ?

Best Regards,
Hou zj

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