Re: Minimal logical decoding on standbys

On 2019-May-23, Andres Freund wrote:

> On 2019-05-23 09:37:50 -0400, Robert Haas wrote:
> > On Thu, May 23, 2019 at 9:30 AM Sergei Kornilov <sk(at)zsrv(dot)org> wrote:
> > > > wal_level is PGC_POSTMASTER.
> > >
> > > But primary can be restarted without restart on standby. We require wal_level replica or highter (currently only logical) on standby. So online change from logical to replica wal_level is possible on standby's controlfile.
> >
> > That's true, but Amit's scenario involved a change in wal_level during
> > the execution of pg_create_logical_replication_slot(), which I think
> > can't happen.
>
> I don't see why not - we're talking about the wal_level in the WAL
> stream, not the setting on the standby. And that can change during the
> execution of pg_create_logical_replication_slot(), if a PARAMTER_CHANGE
> record is replayed. I don't think it's actually a problem, as I
> outlined in my response to Amit, though.

I don't know if this is directly relevant, but in commit_ts.c we go to
great lengths to ensure that things continue to work across restarts and
changes of the GUC in the primary, by decoupling activation and
deactivation of the module from start-time initialization. Maybe that
idea is applicable for this too?

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Álvaro Herrera https://www.2ndQuadrant.com/
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