Re: [HACKERS] [PATCH] Vacuum: Update FSM more frequently

On Sat, Mar 24, 2018 at 4:17 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Claudio Freire <klaussfreire(at)gmail(dot)com> writes:
>> [ 0001-Vacuum-Update-FSM-more-frequently-v9.patch ]
>
> I hadn't paid any attention to this patch previously, so maybe I'm
> missing something ... but this sure seems like a very bizarre approach
> to the problem. If the idea is to fix the FSM's upper levels after
> vacuuming a known sub-range of the table, why would you not proceed
> by teaching FreeSpaceMapVacuum to recurse only within that sub-range
> of page numbers? This setup with a threshold seems entirely Rube
> Goldbergian. It's dependent on a magic threshold number that you can
> only select by trial and error, and it's inevitably going to spend time
> updating segments of the FSM tree that have nothing to do with the part
> that's been vacuumed.

Well, the point is to not only update the range we know we've
vacuumed, but also to finish the updates done by a potential
previously cancelled autovacuum.

But I guess that's only for the first run. Your approach seems like a
better idea for the other runs.

Just FTR, the number isn't so magic. During vacuum, the patch records
the highest amount of free space produced, and will only recurse into
branches that don't already record that much free space. So it's quite
deterministic in those cases, it's only the first run the one that has
to guess, and your approach doesn't apply for that first run.