Re: Lowering the ever-growing heap->pd_lower

On Fri, Apr 8, 2022 at 6:17 AM Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> I agree that the value of 291 is pretty much accidental, but it also
> seems fairly generous to me. The bigger you make it, the more space
> you can waste. I must have missed (or failed to understand) previous
> discussions about why raising it would be a good idea.

What do you mean about wasting space? Wasting space on the stack? I
can't imagine you meant wasting space on the page, since being able to
accomodate more items on each heap page seems like it would be
strictly better, barring any unintended weird FSM issues.

As far as I know the only real downside to increasing it is the impact
on tidbitmap.c. Increasing the number of valid distinct TID values
might have a negative impact on performance during bitmap scans, which
will need to be managed. However, I don't think that increased stack
space usage will be a problem, with a little work. It either won't
matter at all (e.g. an array of offset numbers on the stack still
won't be very big), or it can be fixed locally where it turns out to
matter (like in lazy_scan_prune).

We used to routinely use MaxOffsetNumber for arrays of item offset
numbers. I cut down on that in the B-Tree code, reducing it to
MaxIndexTuplesPerPage (which is typically 407) in a few places. So
anything close to our current MaxIndexTuplesPerPage ought to be fine
for most individual arrays stored on the stack.

--
Peter Geoghegan