On Tue, Mar 29, 2022 at 5:01 PM Andres Freund <and...@anarazel.de> wrote: > I think it's reasonably rare because in cases there'd be corruption, we'd > typically not even have written them out / throw them away explicitly - we > only read stats when starting without crash recovery. > > So the "expected" case of corruption afaicts solely is a OS crash just after > the shutdown checkpoint completed?
Can we prevent that case from occurring, so that there are no expected cases? > > And maybe we should have, inside the stats system, something that > > keeps track of when the stats file was last recreated from scratch because > > of a corruption event, separately from when it was last intentionally reset. > > That would be easy to add. Don't think we have a good place to show the > information right now - perhaps just new functions not part of any view? I defer to you on where to put it. > I can think of these different times: > > - Last time stats were removed due to starting up in crash recovery > - Last time stats were created from scratch, because no stats data file was > present at startup > - Last time stats were thrown away due to corruption > - Last time a subset of stats were reset using one of the pg_reset* functions > > Makes sense? Yes. Possibly that last could be broken in to two: when all stats were last reset, when some stats were last reset. > > Does redo update the stats? > > With "update" do you mean generate new stats? In the shared memory stats patch > it triggers stats to be dropped, on HEAD it just resets all stats at startup. > > Redo itself doesn't generate stats, but bgwriter, checkpointer, backends do. Well, I guess what I'm trying to figure out is what happens if we run in recovery for a long time -- say, a year -- and then get promoted. Do we have reasons to expect that the stats will be accurate enough to use at that point, or will they be way off? I don't have a great understanding of how this all works, but if running recovery for a long time is going to lead to a situation where the stats progressively diverge from reality, then preserving them doesn't seem as valuable as if they're going to be more or less accurate. -- Robert Haas EDB: http://www.enterprisedb.com
