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

On Thu, Oct 23, 2025 at 2:53 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>
> On Wed, Oct 22, 2025 at 4:06 AM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:
> >
> > On Wed, Oct 22, 2025 at 2:58 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > >
> > > On Wed, Oct 22, 2025 at 2:40 PM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:
> > > >
> > > > On Fri, Oct 17, 2025 at 11:09 PM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> > > > >
> > > >
> > > > I want to discuss the create-publication case, which currently gives
> > > > this warning:
> > > >
> > > > postgres=# create publication pub1 for all tables;
> > > > WARNING: logical decoding should be allowed to publish logical changes
> > > > HINT: Before creating subscriptions, set "wal_level" >= "logical" or
> > > > create a logical replication slot when "wal_level" = "replica".
> > > > CREATE PUBLICATION
> > > >
> > > > But is this warning really necessary during publication creation?
> > > >
> > >
> > > On HEAD, the WARNING/HINT is as follows for the above case:
> > > WARNING: "wal_level" is insufficient to publish logical changes
> > > HINT: Set "wal_level" to "logical" before creating subscriptions.
> > >
> > > Shouldn't we simply change the HINT to "Set "wal_level" to "logical"
> > > or create a logical replication slot before creating subscriptions."?
> > >
> >
> > My point was do we really need this WARNING considering case a) does
> > not need it now and case b) also gives different ERROR (cases
> > mentioned in previous email). But if we plan to retain the WARNING,
> > then I agree with the change proposed here, it looks simpler and
> > better.
>
> I guess I understand your point. Without the patch, the WARNING
> message makes sense because if users create a publication with
> wal_level='replica', they need to change wal_level to 'logical' to
> start logical replication. That is, it works like telling users some
> required operations that they might have missed. On the other hand,
> with the patch, logical decoding is automatically enabled when they
> start logical replication, and any additional step is no longer
> necessary.

Right.

> I find the message makes less sense now but is still
> required in 'minimal' WAL level cases?
>

Yes, it is needed in 'minimal' wal-level case. We shall change the
check in CreatePublication() then.

thanks
Shveta