Hi, On 2021-06-16 09:46:07 -0700, Peter Geoghegan wrote: > On Wed, Jun 16, 2021 at 9:03 AM Peter Geoghegan <p...@bowt.ie> wrote: > > On Wed, Jun 16, 2021 at 3:59 AM Matthias van de Meent > > > So the implicit assumption in heap_page_prune that > > > HeapTupleSatisfiesVacuum(OldestXmin) is always consistent with > > > heap_prune_satisfies_vacuum(vacrel) has never been true. In that case, > > > we'll need to redo the condition in heap_page_prune as well. > > > > I don't think that this shows that the assumption within > > lazy_scan_prune() (the assumption that both "satisfies vacuum" > > functions agree) is wrong, with the obvious exception of cases > > involving the bug that Justin reported. GlobalVis*.maybe_needed is > > supposed to be conservative. > > I suppose it's true that they can disagree because we call > vacuum_set_xid_limits() to get an OldestXmin inside vacuumlazy.c > before calling GlobalVisTestFor() inside vacuumlazy.c to get a > vistest. But that only implies that a tuple that would have been > considered RECENTLY_DEAD inside lazy_scan_prune() (it just missed > being considered DEAD according to OldestXmin) is seen as an LP_DEAD > stub line pointer. Which really means it's DEAD to lazy_scan_prune() > anyway. These days the only way that lazy_scan_prune() can consider a > tuple fully DEAD is if it's no longer a tuple -- it has to actually be > an LP_DEAD stub line pointer.
I think it's more complicated than that - "before" isn't a guarantee when the horizon can go backwards. Consider the case where a hot_standby_feedback=on replica without a slot connects - that can result in the xid horizon going backwards. I think a good way to address this might be to have GlobalVisUpdateApply() ensure that maybe_needed does not go backwards within one backend. This is *nearly* already guaranteed within vacuum, except for the case where a catalog access between vacuum_set_xid_limits() and GlobalVisTestFor() could lead to an attempt at pruning, which could move maybe_needed to go backwards theoretically if inbetween those two steps a replica connected that causes the horizon to go backwards. Greetings, Andres Freund
