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.
--
Peter Eisentraut http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
