On Mon, 21 Nov 2022 at 15:23, Robert Haas <robertmh...@gmail.com> wrote: > > On Mon, Nov 21, 2022 at 10:14 AM Simon Riggs > <simon.ri...@enterprisedb.com> wrote: > > > > What we need is a solution that avoids reading an unbounded number of > > > > tuples under any circumstances. I previously suggested using > > > > SnapshotAny here, but Tom didn't like that. I'm not sure if there are > > > > safety issues there or if Tom was just concerned about the results > > > > being misleading. Either way, maybe there's some variant on that theme > > > > that could work. For instance, could we teach the index scan to stop > > > > if the first 100 tuples that it finds are all invisible? Or to reach > > > > at most 1 page, or at most 10 pages, or something? > > > > > > A hard limit on the number of index pages examined seems like it > > > might be a good idea. > > > > Good, that is what the patch does. > > <looks at patch> > > Oh, that's surprisingly simple. Nice! > > Is there any reason to tie this into page costs? I'd be more inclined > to just make it a hard limit on the number of pages. I think that > would be more predictable and less prone to surprising (bad) behavior. > And to be honest I would be inclined to make it quite a small number. > Perhaps 5 or 10. Is there a good argument for going any higher?
+1, that makes the patch smaller and the behavior more predictable. (Just didn't want to do anything too simple, in case it looked like a kluge.) -- Simon Riggs http://www.EnterpriseDB.com/
