On Tue, Jun 9, 2020 at 1:20 PM Amit Kapila <amit.kapil...@gmail.com> wrote:
> On Tue, Jun 9, 2020 at 2:31 PM Magnus Hagander <mag...@hagander.net> > wrote: > > > > On Tue, Jun 9, 2020 at 10:53 AM Kyotaro Horiguchi < > horikyota....@gmail.com> wrote: > >> > >> At Tue, 9 Jun 2020 08:52:24 +0200, Peter Eisentraut < > peter.eisentr...@2ndquadrant.com> wrote in > >> > 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. > >> > >> Currently logical slots cannot be created while wal_level < > >> logical. Thus a database that has a logical slot must have been once > >> executed with wal_level >= logical before the creation of the slot. > >> > > I think the creation of slot would take a lot more time in that case > as it needs to wait for existing transactions to finish which I feel > could be confusing to users. Sure, the cost would have to be incurred > the first time but still the user might tempt to cancel such an > operation if he is not aware of the internals. > Yeah, I am unsure if this is doable, but I think that's what Peter was trying to explain, because that's what would be most user-friendly. But it may definitely not be worth the complexity, I'm guessing. Being able to change wal_level on reload instead of restart would be less user friendly than that, but more user friendly than what we have now. -- Magnus Hagander Me: https://www.hagander.net/ <http://www.hagander.net/> Work: https://www.redpill-linpro.com/ <http://www.redpill-linpro.com/>
