Re: Teaching users how they can get the most out of HOT in Postgres 14

On Mon, Apr 12, 2021 at 04:35:13PM -0700, Peter Geoghegan wrote:
> On Mon, Apr 12, 2021 at 4:30 PM Andres Freund <andres(at)anarazel(dot)de> wrote:
> > As far as I can see there's no reasonable way to disable this
> > "optimization", which scares me.
>
> I'm fine with adding a simple 'off' switch. What I'd like to avoid
> doing is making the behavior tunable, since it's likely to change in
> Postgres 15 and Postgres 16 anyway.

While going through this commit a couple of days ago, I really got to
wonder why you are controlling this stuff with a hardcoded value and I
found that scary, while what you should be using are two GUCs with the
reloptions that come with the feature (?):
- A threshold, as an integer, to define a number of pages.
- A scale factor to define a percentage of pages.

Also, I am a bit confused with the choice of BYPASS_THRESHOLD_PAGES as
parameter name. For all the other parameters of autovacuum, we use
"threshold" for a fixed number of items, not a percentage of a given
item.
--
Michael