Re: Combine Prune and Freeze records emitted by vacuum

On Wed, Mar 27, 2024 at 2:26 PM Melanie Plageman
<melanieplageman@gmail.com> wrote:
>
> On Wed, Mar 27, 2024 at 12:18 PM Heikki Linnakangas <hlinnaka@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);

sorry, that should say presult->htsv[rootoffnum] != HEAPTUPLE_DEAD.

The latter should be a subset of the former. But instead it seems
there are cases I missed by doing only the former.

- Melanie