On Thu, Jul 19, 2018 at 4:29 AM, Masahiko Sawada <sawada.m...@gmail.com> wrote: >> One area that might be worth investigating is retail index tuple >> deletion performed within the executor in the event of non-HOT >> updates. Maybe LP_REDIRECT could be repurposed to mean "ghost record", >> at least in unique index tuples with no NULL values. The idea is that >> MVCC index scans can skip over those if they've already found a >> visible tuple with the same value. > > I think that's a good idea. The overhead of marking it as ghost seems > small and it would speed up index scans. If MVCC index scans have > already found a visible tuples with the same value they can not only > skip scanning but also kill them? If can, we can kill index tuples > without checking the heap.
I think you're talking about making LP_REDIRECT marking in nbtree represent a "recently dead" hint: the deleting transaction has committed, and so we are 100% sure that the tuple is about to become garbage, but it cannot be LP_DEAD just yet because it needs to stay around for the benefit of at least one existing snapshot/long running transaction. That's a different idea to what I talked about, since it could perhaps work in a way that's much closer to LP_DEAD/kill_prior_tuple (no extra executor stuff is required). I'm not sure if your idea is better or worse than what I suggested, though. It would certainly be easier to implement. -- Peter Geoghegan
