Hi, On 2023-04-25 16:00:24 -0400, Robert Haas wrote: > On Tue, Apr 25, 2023 at 2:39 PM Andres Freund <[email protected]> wrote: > > I'm mildly inclined to not consider it a bug, given that this looks to have > > been true for other stats for quite a while? But it does still seem worth > > improving upon - I'd make the consideration when to apply the relevant > > patches > > depend on the complexity. I'm worried we'd need to introduce sufficiently > > new > > infrastructure that 16 doesn't seem like a good idea. Let's come up with a > > patch and judge it after? > > ISTM that it's pretty desirable to do something about this. If the > process isn't going to report statistics properly, at least remove it > from the view.
It's populated after crash recovery, when shutting down and at the time of promotion, that isn't *completely* crazy. > If it can be made to report properly, that would be even better. But > shipping a new view with information that will nearly always be zeroes > instead of real data seems like a bad call, even if there are existing cases > that have the same problem. I refreshed my memory: The startup process has indeed behaved that way for much longer than pg_stat_io existed - but it's harder to spot, because the stats are more coarsely aggregated :/. And it's very oddly inconsistent: The startup process doesn't report per-relation read/hit (it might when we create a fake relcache entry, to lazy to see what happens exactly), because we key those stats by oid. However, it *does* report the read/write time. But only at process exit, of course. The weird part is that the startup process does *NOT* increase pg_stat_database.blks_read/blks_hit, because instead of basing those on pgBufferUsage.shared_blks_read etc, we compute them based on the relation level stats. pgBufferUsage is just used for EXPLAIN. This isn't recent, afaict. TL;DR: Currently the startup process maintains blk_read_time, blk_write_time, but doesn't maintain blks_read, blks_hit - which doesn't make sense. Yikes. Greetings, Andres Freund
