Re: Removing more vacuumlazy.c special cases, relfrozenxid optimizations

On Fri, Feb 11, 2022 at 8:30 PM Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
> Attached is v8. No real changes -- just a rebased version.

Concerns about my general approach to this project (and even the
Postgres 14 VACUUM work) were expressed by Robert and Andres over on
the "Nonrandom scanned_pages distorts pg_class.reltuples set by
VACUUM" thread. Some of what was said honestly shocked me. It now
seems unwise to pursue this project on my original timeline. I even
thought about shelving it indefinitely (which is still on the table).

I propose the following compromise: the least contentious patch alone
will be in scope for Postgres 15, while the other patches will not be.
I'm referring to the first patch from v8, which adds dynamic tracking
of the oldest extant XID in each heap table, in order to be able to
use it as our new relfrozenxid. I can't imagine that I'll have
difficulty convincing Andres of the merits of this idea, for one,
since it was his idea in the first place. It makes a lot of sense,
independent of any change to how and when we freeze.

The first patch is tricky, but at least it won't require elaborate
performance validation. It doesn't change any of the basic performance
characteristics of VACUUM. It sometimes allows us to advance
relfrozenxid to a value beyond FreezeLimit (typically only possible in
an aggressive VACUUM), which is an intrinsic good. If it isn't
effective then the overhead seems very unlikely to be noticeable. It's
pretty much a strictly additive improvement.

Are there any objections to this plan?

--
Peter Geoghegan