Re: Emit fewer vacuum records by reaping removable tuples during pruning

On Wed, Jan 17, 2024 at 4:25 PM Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
>
> On Wed, Jan 17, 2024 at 3:58 PM Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> > > Ah, I realize I was not clear. I am now talking about inconsistencies
> > > in vacuuming the FSM itself. FreeSpaceMapVacuumRange(). Not updating
> > > the freespace map during the course of vacuuming the heap relation.
> >
> > Fair enough, but I'm still not quite sure exactly what the question
> > is. It looks to me like the current code, when there are indexes,
> > vacuums the FSM after each round of index vacuuming. When there are no
> > indexes, doing it after each round of index vacuuming would mean never
> > doing it, so instead we vacuum the FSM every ~8GB. I assume what
> > happened here is that somebody decided doing it after each round of
> > index vacuuming was the "right thing," and then realized that was not
> > going to work if no index vacuuming was happening, and so inserted the
> > 8GB threshold to cover that case.
>
> Note that VACUUM_FSM_EVERY_PAGES is applied against the number of
> rel_pages "processed" so far -- *including* any pages that were
> skipped using the visibility map. It would make a bit more sense if it
> was applied against scanned_pages instead (just like
> FAILSAFE_EVERY_PAGES has been since commit 07eef53955). In other
> words, VACUUM_FSM_EVERY_PAGES is applied against a thing that has only
> a very loose relationship with physical work performed/time elapsed.

This is a good point. Seems like a very reasonable change to make, as
I would think that was the original intent.

- Melanie