Hi, On 2021-04-21 14:38:44 +0200, Magnus Hagander wrote: > Andres mentioned at least once over in the thread about shared memory > stats collection that being able to have persistent stats could come > out of that one in the future. Whatever is done on the topic should > probably be done based on that work, as it provides a better starting > point and also one that will stay around.
Yea. I think the main benefit from the shared memory stats patch that would make this a easier is that it tracks (with one small hole that can probably be addressed) dropped objects accurately, even across crashes / replication. Having old stats around runs into danger of mixing stats for an old dropped object being combined with stats for a new object. I don't think making pgstat.c fully durable by continually storing the data in a table or something like that is an option. For one, the stats for a replica and primary are independent. For another, the overhead would be prohibitive. But after we gain the accurate dropping of stats we can store a stats snapshot corresponding to certain WAL records (by serializing to something like pg_stats_%redo_lsn%) without ending up with dropped stats surviving. A big question around this is how often we'd want to write out the stats. Obviously, the more often we do, the higher the overhead. And the less frequently, the more stats updates might be lost. Patrik, for your use cases, would loosing at most the stats since the start of last checkpoint be an issue? Greetings, Andres Freund
