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Re: Adding REPACK [concurrently]

Alvaro Herrera <alvherre@alvh.no-ip.org>

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From: Alvaro Herrera <alvherre@alvh.no-ip.org>
To: Antonin Houska <ah@cybertec.at>
Cc: Srinath Reddy Sadipiralla <srinath2133@gmail.com>, Matthias van de Meent <boekewurm+postgres@gmail.com>, Mihail Nikalayeu <mihailnikalayeu@gmail.com>, Pg Hackers <pgsql-hackers@lists.postgresql.org>, Robert Treat <rob@xzilla.net>
Date: 2026-03-20T07:59:23Z
Lists: pgsql-hackers 
On 2026-Mar-19, Antonin Houska wrote:

> Alvaro Herrera <alvherre@alvh.no-ip.org> wrote:
> 
> > So here's v43.  Here, I've changed the CONCURRENTLY implementation to go
> > through table AM.  This necessitated changing it to use tuples in slots
> > instead of HeapTuple.  This is good because we can avoid repeated tuple
> > form/deform, which could get pretty expensive.  Antonin's 0004 patch
> > here looks suspicious here though, because it deforms the tuple and
> > forms it again, which sounds unnecessary now.
> 
> I suppose you mean
> v42-0004-Serialize-decoded-tuples-without-flattening.patch. This deforms the
> tuple to get the external attributes and to write them to file. The tuple the
> logical worker received from reorderbuffer.c cannot be passed to the backend
> executing REPACK because it may contain "external indirect" attributes,
> i.e. pointers to the worker's memory.

No, that patch has been absorbed in what is now v43-0003.  I meant
v43-0004 "Use BulkInsertState when copying data to the new heap.",
that's why I said "patch 0004 here".  In this patch, we have
reform_tuple which deforms the tuple, sets to NULL any attribute that's
marked dropped, and then forms a new one.  This is wasteful and should
probably be done elsewhere, while the tuple is still in slot
representation.  In fact, I suspect it may not be necessary at all
anymore.

I haven't verified whether all the code is covered by existing tests;
what I did was just run them.  But to ensure that it is all trustworthy,
I'll spend some time with the coverage report to ensure there aren't any
nasty surprises anywhere.  The slot/tupdesc interface is notoriously bad
at differentiating 0-based indexes of the attribute array, and 1-based
proper attribute numbers, so it's very easy to do the wrong thing.
(It's worse when you do an even number of wrong things and they cancel
each other out.)

-- 
Álvaro Herrera        Breisgau, Deutschland  —  https://www.EnterpriseDB.com/
"This is what I like so much about PostgreSQL.  Most of the surprises
are of the "oh wow!  That's cool" Not the "oh shit!" kind.  :)"
Scott Marlowe, http://archives.postgresql.org/pgsql-admin/2008-10/msg00152.php

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