Re: Lowering the ever-growing heap->pd_lower

On Wed, 4 Aug 2021 at 01:43, Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
>
> On Mon, Aug 2, 2021 at 11:57 PM Simon Riggs
> <simon(dot)riggs(at)enterprisedb(dot)com> wrote:
> > 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.
>
> Why do you say that?

Truncating line pointers can make extra space on the page, so it could
be the difference between a HOT and a non-HOT update. My understanding
is that these just-in-time actions have a beneficial effect in other
circumstances, so we can do that here also.

If we truncate line pointers more frequently then the root pointers
will tend to be lower in the array, which will make truncation even
more effective.

> > 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?
>
> What I meant before was that this argument worked back when we limited
> the technique to VACUUM's second heap pass. Doing line pointer array
> truncation at that point alone meant that it only ever happened
> outside of VACUUM proper. Prior to that point we literally did nothing
> about LP_UNUSED items at the end of each line pointer array, so we
> were going from doing nothing to doing something.
>
> 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.

If the only thing we do to a page is truncate line pointers then it
may not be worth it. But dirtying a page to reclaim space is also the
precise time when reclaiming line pointers makes sense also. So if the
page is dirtied by cleaning, then that is the time to reclaim line
pointers also.

Again, why would we reclaim space to avoid bloat but then ignore any
line pointer bloat?

It's not clear why truncating unused line pointers would break DDL.

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