On 8/15/25 01:05, Peter Geoghegan wrote: > On Thu, Aug 14, 2025 at 6:24 PM Tomas Vondra <to...@vondra.me> wrote: >> FWIW I'm not claiming this explains all odd things we're investigating >> in this thread, it's more a confirmation that the scan direction may >> matter if it translates to direction at the device level. I don't think >> it can explain the strange stuff with the "random" data sets constructed >> Peter. > > The weird performance characteristics of that one backwards scan are > now believed to be due to the WaitIO issue that Andres described about > an hour ago. That issue seems unlikely to only affect backwards > scans/reverse-sequential heap I/O. >
Good. I admit I lost track of which the various regressions may affect existing plans, and which are specific to the prefetch patch. > I accept that backwards scans are likely to be significantly slower > than forwards scans on most/all SSDs. But that in itself doesn't > explain why the same issue didn't cause the equivalent sequential > forward scan to also be a lot slower. Actually, it probably *did* > cause that forwards scan to be *somewhat* slower -- just not by enough > to immediately jump out at me (not enough to make the forwards scan > much slower than a scan that does wholly random I/O, which is > obviously absurd). > True. That's weird. > My guess is that once we fix the underlying problem, we'll see > improved performance for many different types of queries. Not as big > of a benefit as the one that the broken query will get, but still > enough to matter. > Hopefully. Let's see. regards -- Tomas Vondra
