From: Antonin Houska <ah@cybertec.at>
To: Andres Freund <andres@anarazel.de>
cc: Alvaro Herrera <alvherre@alvh.no-ip.org>,
    Amit Kapila <amit.kapila16@gmail.com>,
    Mihail Nikalayeu <mihailnikalayeu@gmail.com>,
    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>
Subject: Re: Adding REPACK [concurrently]
In-reply-to: <cdgw4sbbfcgk6du3iv54r2dgiy4tfywoklbotlmj4irxavdcr3@glxfw5jj277q>
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 <cdgw4sbbfcgk6du3iv54r2dgiy4tfywoklbotlmj4irxavdcr3@glxfw5jj277q>
Comments: In-reply-to Andres Freund <andres@anarazel.de>
   message dated "Tue, 07 Apr 2026 10:10:34 -0400."
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Date: Tue, 07 Apr 2026 17:38:24 +0200
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Andres Freund <andres@anarazel.de> wrote:

> On 2026-04-07 14:33:50 +0200, Alvaro Herrera wrote:
> > On 2026-Apr-07, Amit Kapila wrote:
> > =

> > > I have a question based on 0001's commit message: "This patch adds a
> > > new option to logical replication output plugin, to declare that it
> > > does not use shared catalogs (i.e. catalogs that can be changed by
> > > transactions running in other databases in the cluster).". In which
> > > cases, currently plugin needs to access multi-database transactions =
or
> > > transactions that need to access shared catalogs and on what basis a
> > > plugin can decide that the changes it requires won't need any such
> > > access.
> > =

> > I don't think any plugin needs "multi-database" access as such, but
> > needing access to shared catalogs is likely normal.  Repack knows it
> > won't access any shared catalogs, so it can set the flag at ease.
> > =

> > There's a cross-check added in the commit that tests for access to
> > shared catalogs if the flag is set to false.  I guess you could set it
> > to false and see what breaks :-)
> =

> I think this has a quite high chance of indirect breakages. You just nee=
d some
> cache invalidation processing / building accessing shared catalogs to vi=
olate
> the rule, and whether that happens very heavily depends on what cache en=
tries
> are present and whether something registers relcache callbacks or such.
> =

> This can be triggered by an output function during logical decoding or s=
uch,
> so you don't really have control over it.

The REPACK plugin only deforms tuples and writes them to a file, so I thin=
k
that things like this should not happen. However, I admit that an option t=
hat
allows the plugin developer to declare "I don't need shared catalogs" may =
be
considered deceptive.

-- =

Antonin Houska
Web: https://www.cybertec-postgresql.com