Re: find replication slots that "belong" to a publication

On Fri, Apr 4, 2025 at 4:58 AM Willy-Bas Loos <willybas(at)gmail(dot)com> wrote:

> Hi!
>
> I'm looking for a way to find out if there are still replication slots
> active for a publication before dropping the publication in an automated
> way. The idea is that the publication is thought not to be needed any
> longer, but we want to make sure.
>
> I'm having trouble finding a link between a publication, the subscriptions
> and the replication slots. Especially when you don't want to make
> assumptions about any subscriber nodes, so you are restricted to the
> publisher node.
>
> The best I could find was a query listed in pg_stat_activity that lists
> the slot name and the publication name:
> START_REPLICATION SLOT "my_slot" LOGICAL 5DD1/3E56D360 (proto_version '1',
> publication_names '"my_publication"')
>
> I don't like the idea of using string manipulation on such query strings
> to get the information I need. Postgres must have a way to compose this
> query.
> Can anyone tell me a way to find replication slots that belong to a
> publication?
>
> --
> Willy-Bas Loos
>

Hi Willy-Bas,

Logical replication slots appear in the views pg_stat_replication and
pg_replication_slots. Both views have the information you are looking for,
the difference is pg_stat_replication shows only the active slots. Keep in
mind Temporary Slots only live for the length of the session that created
it; the slot will appear in both views.

The bigger issue I think you are trying to address is when can a slot be
dropped safely. Once a logical replication slot is dropped there is no
recovery of the slot's lsn position. Probably the best way to decide if a
slot has been abandoned is how far behind it is. The pg_wal_lsn_diff can
be used to figure out how far behind a slot is

https://www.postgresql.org/docs/13/view-pg-replication-slots.html
https://www.postgresql.org/docs/13/monitoring-stats.html#MONITORING-PG-STAT-REPLICATION-VIEW

https://www.postgresql.org/docs/current/functions-admin.html#FUNCTIONS-ADMIN-BACKUP

Hope this answers your question

Justin