On Wed, Mar 27, 2024 at 12:18 PM Heikki Linnakangas <hlinn...@iki.fi> wrote: > > On 27/03/2024 17:18, Melanie Plageman wrote: > > I need some way to modify the control flow or accounting such that I > > know which HEAPTUPLE_RECENTLY_DEAD tuples will not be marked LP_DEAD. > > And a way to consider freezing and do live tuple accounting for these > > and HEAPTUPLE_LIVE tuples exactly once. > > Just a quick update: I've been massaging this some more today, and I > think I'm onto got something palatable. I'll send an updated patch later > today, but the key is to note that for each item on the page, there is > one point where we determine the fate of the item, whether it's pruned > or not. That can happen in different points in in heap_page_prune(). > That's also when we set marked[offnum] = true. Whenever that happens, we > all call one of the a heap_page_prune_record_*() subroutines. We already > have those subroutines for when a tuple is marked as dead or unused, but > let's add similar subroutines for the case that we're leaving the tuple > unchanged. If we move all the bookkeeping logic to those subroutines, we > can ensure that it gets done exactly once for each tuple, and at that > point we know what we are going to do to the tuple, so we can count it > correctly. So heap_prune_chain() decides what to do with each tuple, and > ensures that each tuple is marked only once, and the subroutines update > all the variables, add the item to the correct arrays etc. depending on > what we're doing with it.
Yes, this would be ideal. I was doing some experimentation with pageinspect today (trying to find that single place where live tuples fates are decided) and it seems like a heap-only tuple that is not HOT-updated will usually be the one at the end of the chain. Which seems like it would be covered by adding a record_live() type function call in the loop of heap_prune_chain(): /* * If the tuple is not HOT-updated, then we are at the end of this * HOT-update chain. */ if (!HeapTupleHeaderIsHotUpdated(htup)) { heap_prune_record_live_or_recently_dead(dp, prstate, offnum, presult); break; } but that doesn't end up producing the same results as if (HeapTupleHeaderIsHeapOnly(htup) && !HeapTupleHeaderIsHotUpdated(htup) && presult->htsv[rootoffnum] == HEAPTUPLE_DEAD) heap_prune_record_live_or_recently_dead(dp, prstate, offnum, presult); at the top of heap_prune_chain(). - Melanie