Re: New strategies for freezing, advancing relfrozenxid early

On Wed, Jan 25, 2023 at 5:15 PM Andres Freund <andres(at)anarazel(dot)de> wrote:
> However, it significantly increases the overall work when rows have a somewhat
> limited lifetime. The documented reason why vacuum_freeze_min_age exist -
> although I think it doesn't really achieve its documented goal anymore, after
> the recent changes page-level freezing changes.

Huh? vacuum_freeze_min_age hasn't done that, at all. At least not
since the visibility map went in back in 8.4:

https://wiki.postgresql.org/wiki/Freezing/skipping_strategies_patch:_motivating_examples#Today.2C_on_Postgres_HEAD_2

That's why we literally do ~100% of all freezing in aggressive mode
VACUUM with append-only or append-mostly tables.

> > VACUUM determines its freezing strategy based on the value of the new
> > vacuum_freeze_strategy_threshold GUC (or reloption) with logged tables;
> > tables that exceed the size threshold use the eager freezing strategy.
>
> I think that's not a sufficient guard at all. The size of a table doesn't say
> much about how a table is used.

Sufficient for what purpose?

> > Eager freezing is strictly more aggressive than lazy freezing. Settings
> > like vacuum_freeze_min_age still get applied in just the same way in
> > every VACUUM, independent of the strategy in use. The only mechanical
> > difference between eager and lazy freezing strategies is that only the
> > former applies its own additional criteria to trigger freezing pages.
>
> That's only true because vacuum_freeze_min_age being has been fairly radically
> redefined recently.

So? This part of the commit message is a simple statement of fact.

--
Peter Geoghegan