On Wed, Aug 25, 2021 at 5:23 PM Alvaro Herrera <alvhe...@alvh.no-ip.org> wrote: > Ooh, this was illuminating -- thanks for explaining. TBH I would have > been very confused if asked to explain what that log line meant; and now > that I know what it means, I am even more convinced that we need to work > harder at it :-) > > I'll see if I can come up with something ...
BTW, I wonder if you need to reconsider PROGRESS_VACUUM_NUM_DEAD_TUPLES in light of all this. It actually counts LP_DEAD items, which aren't really dead tuples. As my example shows, the distinction between "tuples removed" (as this log output refers to them) and LP_DEAD items removed from heap pages can be very important. One way of handling this might be to call LP_DEAD items "items removed from indexes" -- "tuples removed" can be treated as "items removed from table". Or something along those lines, at least. This is how I phrase it in certain vacuumlazy.c source code comments already. It's not 100% accurate, but in a way it's a lot closer to the truth. And it allows you to sidestep the issue with PROGRESS_VACUUM_NUM_DEAD_TUPLES by only slightly redefining what that means to users -- it can be recast as information about index tuples specifically (there may not actually be any matching index tuples, especially in Postgres 14, but that isn't worth getting in to in user docs IMV). -- Peter Geoghegan
