On Sat, Apr 24, 2021 at 12:56 PM Andres Freund <and...@anarazel.de> wrote: > Did anybody actually argue for using #live entries directly? I think > *dead* entries is more relevant, partiuclarly because various forms of > local cleanup can be taken into account. Live tuples might come in to > put the number of dead tuples into perspective, but otherwise not that > much?
I was unclear. I can't imagine how you'd do anything like this without using both together. Or if you didn't use live tuples you'd use heap blocks instead. Something like that. > > There are many cases where this will do completely the wrong thing, > > even if we have perfectly accurate information. > > Imo the question isn't really whether criteria will ever do something > wrong, but how often and how consequential such mistakes will > be. E.g. unnecessarily vacuuming an index isn't fun, but it's better > than ending up not never cleaning up dead index pointers despite repeat > accesses (think bitmap scans). I strongly agree. The risk with what I propose is that we'd somehow overlook a relevant extreme cost. But I think that that's an acceptable risk. Plus I see no workable alternative -- your "indexes that insert on one end, delete from the other" example works much better as an argument against what you propose than an argument against my own alternative proposal. Which reminds me: how would your framework for index bloat/skipping indexes in VACUUM deal cope with this same scenario? Though I don't think that it's useful to use quantitative thinking as a starting point here, that doesn't mean there is exactly zero role for it. Not sure about how far I'd go here. But I would probably not argue that we shouldn't vacuum an index that is known to (say) be more than 60% dead tuples. I guess I'd always prefer to have a better metric, but speaking hypothetically: Why take a chance? This is not because it's definitely worth it -- it really isn't! It's just because the benefit of being right is low compared to the cost of being wrong -- as you point out, that is really important. -- Peter Geoghegan
