Re: Lowering the ever-growing heap->pd_lower

On Tue, 18 May 2021 at 20:33, Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
>
> On Tue, May 18, 2021 at 12:29 PM Matthias van de Meent
> <boekewurm+postgres(at)gmail(dot)com> wrote:
> > PFA the updated version of this patch. Apart from adding line pointer
> > truncation in PageRepairFragmentation (as in the earlier patches), I
> > also altered PageTruncateLinePointerArray to clean up all trailing
> > line pointers, even if it was the last item on the page.
>
> Can you show a practical benefit to this patch, such as an improvement
> in throughout or in efficiency for a given workload?
>
> It was easy to see that having something was better than having
> nothing at all. But things are of course different now that we have
> PageTruncateLinePointerArray().

There does seem to be utility in Matthias' patch, which currently does
two things:
1. Allow same thing as PageTruncateLinePointerArray() during HOT cleanup
That is going to have a clear benefit for HOT workloads, which by
their nature will use a lot of line pointers.
Many applications are updated much more frequently than they are vacuumed.
Peter - what is your concern about doing this more frequently? Why
would we *not* do this?

2. Reduce number of line pointers to 0 in some cases.
Matthias - I don't think you've made a full case for doing this, nor
looked at the implications.
The comment clearly says "it seems like a good idea to avoid leaving a
PageIsEmpty()" page behind.
So I would be inclined to remove that from the patch and consider that
as a separate issue, or close this.

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