On Tue, Aug 30, 2022 at 9:37 PM Jeff Davis <pg...@j-davis.com> wrote: > On Tue, 2022-08-30 at 18:50 -0700, Peter Geoghegan wrote: > > Since VM snapshots are immutable, it should be relatively > > easy to have the implementation make its final decision on skipping > > only *after* lazy_scan_heap() returns. > > I like this idea.
I was hoping that you would. I imagine that this idea (with minor variations) could enable an approach that's much closer to what you were thinking of: one that mostly focuses on controlling the number of unfrozen pages, and not so much on advancing relfrozenxid early, just because we can and we might not get another chance for a long time. In other words your idea of a design that can freeze more during a non-aggressive VACUUM, while still potentially skipping all-visible pages. I said earlier on that we ought to at least have a strong bias in the direction of advancing relfrozenxid in larger tables, especially when we decide to freeze whole pages more eagerly -- we only get one chance to advance relfrozenxid per VACUUM, and those opportunities will naturally be few and far between. We cannot really justify all this extra freezing if it doesn't prevent antiwraparound autovacuums. That was more or less my objection to going in that direction. But if we can more or less double the number of opportunities to at least ask the question "is now a good time to advance relfrozenxid?" without really paying much for keeping this option open (and I think that we can), my concern about relfrozenxid advancement becomes far less important. Just being able to ask that question is significantly less rare and precious. Plus we'll probably be able to make significantly better decisions about relfrozenxid overall with the "second phase because I changed my mind about skipping" concept in place. -- Peter Geoghegan
