Feedback-ID: ia2694551:Fastmail
Date: Fri, 3 Apr 2026 16:59:09 +0200
From: Alvaro Herrera <alvherre@alvh.no-ip.org>
To: Antonin Houska <ah@cybertec.at>
Cc: Srinath Reddy Sadipiralla <srinath2133@gmail.com>,
	Amit Kapila <amit.kapila16@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>
Subject: Re: Adding REPACK [concurrently]
Message-ID: <202604031448.3nakw63kxkmr@alvherre.pgsql>
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On 2026-Apr-03, Antonin Houska wrote:

> This is an alternative implementation of 0006, allowing one backend running
> REPACK (CONCURRENTLY) in a database, instead of one backend in a cluster.

Thanks!  so I'm removing the previous one and taking this one.  Here's a
v50:

- In testing, I noticed that we could sometimes request a Flush for a
  WAL position that hasn't been written yet.  This is due to my
  replacing the original code that wrote a dummy xlog record that we
  could flush, with a call to XLogGetInsertRecPtr().  So we'd get an
  error like

LOG:  request to flush past end of generated WAL; request 0/15CF0018, current position 0/15CF000
  Antonin promptly noticed that this is because XLogGetInsertRecPtr()
  gets the LSN past the segment header, which is 18 bytes wrong.  So the
  fix here is to use XLogGetInsertEndRecPtr() instead.
  
- My testing also uncovered a problem with exclusion constraints; tables
  with them would fail to repack like
  ERROR:  exclusion constraint record missing for rel temporal_fk_mltrng2mltrng_pk_repacknew
  Antonin sent a patch to create copies of the constraints on the
  transient index, which seems like it fixes the problem.  Those
  constraints are obviously discarded together with the transient index.

- I polished the patch to reserve replication slots for REPACK.  Given
  the new implementation of 0006 that was submitted implies that we can
  now run multiple repacks concurrently, I changed the default of 1 to 5.

-- 
Álvaro Herrera         PostgreSQL Developer  —  https://www.EnterpriseDB.com/
"Here's a general engineering tip: if the non-fun part is too complex for you
to figure out, that might indicate the fun part is too ambitious." (John Naylor)
https://postgr.es/m/CAFBsxsG4OWHBbSDM%3DsSeXrQGOtkPiOEOuME4yD7Ce41NtaAD9g%40mail.gmail.com

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