Re: Lowering the ever-growing heap->pd_lower

On Wed, 16 Feb 2022 at 20:54, Peter Geoghegan <pg@bowt.ie> wrote:
>
> On Tue, Feb 15, 2022 at 10:48 AM Matthias van de Meent
> <boekewurm+postgres@gmail.com> wrote:
> > Peter Geoghegan asked for good arguments for the two changes
> > implemented. Below are my arguments detailed, with adversarial loads
> > that show the problematic behaviour of the line pointer array that is
> > fixed with the patch.
>
> Why is it okay that lazy_scan_prune() still calls
> PageGetMaxOffsetNumber() once for the page, before it ever calls
> heap_page_prune()? Won't lazy_scan_prune() need to reestablish maxoff
> now, if only so that its scan-page-items loop doesn't get confused
> when it goes on to read "former line pointers"? This is certainly
> possible with the CLOBBER_FREED_MEMORY stuff in place (which will
> memset the truncated line pointer space with a 0x7F7F7F7F pattern).

Good catch, it is not. Attached a version that re-establishes maxoff
after each prune operation.

-Matthias