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help / color / mirror / Atom feedFrom: Antonin Houska <ah@cybertec.at>
To: Andres Freund <andres@anarazel.de>
Cc: Alvaro Herrera <alvherre@alvh.no-ip.org>
Cc: Amit Kapila <amit.kapila16@gmail.com>
Cc: Mihail Nikalayeu <mihailnikalayeu@gmail.com>
Cc: Srinath Reddy Sadipiralla <srinath2133@gmail.com>
Cc: Matthias van de Meent <boekewurm+postgres@gmail.com>
Cc: Pg Hackers <pgsql-hackers@lists.postgresql.org>
Cc: Robert Treat <rob@xzilla.net>
Subject: Re: Adding REPACK [concurrently]
Date: Tue, 07 Apr 2026 17:38:24 +0200
Message-ID: <227677.1775576304@localhost> (raw)
In-Reply-To: <cdgw4sbbfcgk6du3iv54r2dgiy4tfywoklbotlmj4irxavdcr3@glxfw5jj277q>
References: <CAA4eK1Jg21ODQ7fS2fvN5W_S5kDRhAP5inj3XMRQaa=s-GbYhw@mail.gmail.com>
<202604071230.b5axxf3qna3m@alvherre.pgsql>
<cdgw4sbbfcgk6du3iv54r2dgiy4tfywoklbotlmj4irxavdcr3@glxfw5jj277q>
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 need some
> cache invalidation processing / building accessing shared catalogs to violate
> the rule, and whether that happens very heavily depends on what cache entries
> are present and whether something registers relcache callbacks or such.
>
> This can be triggered by an output function during logical decoding or such,
> so you don't really have control over it.
The REPACK plugin only deforms tuples and writes them to a file, so I think
that things like this should not happen. However, I admit that an option that
allows the plugin developer to declare "I don't need shared catalogs" may be
considered deceptive.
--
Antonin Houska
Web: https://www.cybertec-postgresql.com
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To: pgsql-hackers@postgresql.org
Cc: ah@cybertec.at, andres@anarazel.de, alvherre@alvh.no-ip.org, amit.kapila16@gmail.com, mihailnikalayeu@gmail.com, srinath2133@gmail.com, boekewurm+postgres@gmail.com, pgsql-hackers@lists.postgresql.org, rob@xzilla.net
Subject: Re: Adding REPACK [concurrently]
In-Reply-To: <227677.1775576304@localhost>
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