Re: Adding REPACK [concurrently]

Hi,

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.

Greetings,

Andres Freund