Re: Lowering the ever-growing heap->pd_lower

On Tue, Feb 15, 2022 at 10:48 AM Matthias van de Meent
<boekewurm+postgres(at)gmail(dot)com> wrote:
> # Truncating lp_array during pruning
> ===========================
>
> The following adversarial load grows the heap relation, but with the
> patch the relation keeps its size. The point being that HOT updates
> can temporarily inflate the LP array significantly, and this patch can
> actively combat that issue while we're waiting for the 2nd pass of
> vacuum to arrive.

I am sympathetic to the idea that giving the system a more accurate
picture of how much free space is available on each heap page is an
intrinsic good. This might help us in a few different areas. For
example, the FSM cares about relatively small differences in available
free space *among* heap pages that are "in competition" in
RelationGetBufferForTuple(). Plus we have a heuristic based on
PageGetHeapFreeSpace() in heap_page_prune_opt() to consider.

We should definitely increase MaxHeapTuplesPerPage before too long,
for a variety of reasons that I have talked about in the past. Its
current value is 291 on all mainstream platforms, a value that's
derived from accidental historic details -- which predate HOT.
Obviously an increase in MaxHeapTuplesPerPage is likely to make the
problem that the patch proposes to solve worse. I lean towards
committing the patch now as work in that direction, in fact.

It helps that this patch now seems relatively low risk.

--
Peter Geoghegan