Re: POC: enable logical decoding when wal_level = 'replica' without a server restart

Hi,

On Mon, Jun 23, 2025 at 05:10:37PM +0900, Masahiko Sawada wrote:
> On Thu, Jun 19, 2025 at 6:00 PM Bertrand Drouvot
> <bertranddrouvot(dot)pg(at)gmail(dot)com> wrote:
> >
> > - pg_activate_logical_decoding() is needed only if there is not already a logical
> > slot on the primary
> > - the drop requires the user to think twice if this is the last logical slot
> > - we don't ask the user to create a logical slot if he does not want to use it
> > on the primary
> >
> > Thoughts?
>
> If there is no logical slot on the primary, how can the user disable
> logical decoding that has been enabled via
> pg_activate_logical_decoding()?

I was thinking to keep the pg_deactivate_logical_decoding() API proposed
in this thread.

> Given the discussion so far, it seems we might want to have a
> safeguard to prevent the effective_wal_level from being dropped to
> 'replica' if the last logical slot is accidentally dropped. Another
> idea we can consider is that we automatically increase
> effective_wal_level to 'logical' upon the logical slot creation but
> don't automatically decrease it when dropping the last slot. To
> decrease the effective_wal_level to 'replica', users would need to do
> that explicitly for example using a SQL function,
> pg_disable_logical_decoding().

Yeah that could be an idea (and then we don't add the new wal_level_action
to the drop slot command).

> We might want to have a GUC parameter
> for users to turn on/off this automatic behavior.

You mean a GUC to both automaticly set effective_wal_level to logical at slot creation
and also decrease effective_wal_level to replica if last replication slot is dropped?

Regards,

--
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com