Re: Adding REPACK [concurrently]

Andres Freund <andres(at)anarazel(dot)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