Re: Bump default wal_level to logical

Hi,

On 2020-06-09 08:52:24 +0200, Peter Eisentraut wrote:
> On 2020-06-08 23:32, Andres Freund wrote:
> > On 2020-06-08 13:27:50 -0400, Tom Lane wrote:
> > > If we can allow wal_level to be changed on the fly, I agree that would
> > > help reduce the pressure to make the default setting more expensive.
> > > I don't recall why it's PGC_POSTMASTER right now, but I suppose there
> > > was a reason for that ...
> >
> > There's reasons, but IIRC they're all solvable with reasonable effort. I
> > think most of it boils down to only being able to rely on the new
> > wal_level after a while. For minimal->recovery we basically need a
> > checkpoint started after the change in configuration, and for
> > recovery->logical we need to wait until all sessions have a) read the
> > new config setting b) finished the transaction that used the old
> > setting.
>
> The best behavior from a user's perspective would be if the WAL level
> automatically switched to logical if logical replication slots are present.
> You might not even need 'logical' as an actual value of wal_level anymore,
> you just need to keep a flag in shared memory that records whether at least
> one logical slot exists.

Yea, it'd be good to have that. But you'd need the same type of
coordination mentioned above, no?

Greetings,

Andres Freund