Re: Lowering the ever-growing heap->pd_lower

On Wed, 4 Aug 2021 at 15:39, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>
> On Tue, Aug 3, 2021 at 8:44 PM Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
> > This time it's quite different: we're truncating the line pointer
> > array during pruning. Pruning often occurs opportunistically, during
> > regular query processing. In fact I'd say that it's far more common
> > than pruning by VACUUM. So the chances of line pointer array
> > truncation hurting rather than helping seems higher.
>
> How would it hurt?
>
> It's easy to see the harm caused by not shortening the line pointer
> array when it is possible to do so: we're using up space in the page
> that could have been made free. It's not so obvious to me what the
> downside of shortening it might be. I suppose there is a risk that we
> shorten it and get no benefit, or even shorten it and have to lengthen
> it again almost immediately. But neither of those things really
> matters unless shortening is expensive. If we can piggy-back it on an
> existing WAL record, I don't really see what the problem is.

Hmm, there is no information in WAL to describe the line pointers
being truncated by PageTruncateLinePointerArray(). We just truncate
every time we see a XLOG_HEAP2_VACUUM record and presume it does the
same thing as the original change.

If that is safe, then we don't need to put the truncation on a WAL
record at all, we just truncate after every XLOG_HEAP2_PRUNE record.

If that is not safe... then we have a PG14 bug.

If we do want to see this in WAL, both xl_heap_vacuum and
xl_heap_prune lend themselves to just adding one more OffsetNumber
onto the existing array, to represent the highest offset after
truncation. So we don't need to change the WAL format.

--
Simon Riggs http://www.EnterpriseDB.com/