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

On Fri, Feb 4, 2022 at 10:45 PM Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
> > While I've seen all the above cases triggering anti-wraparound cases
> > by far the majority of the cases are not of these pathological forms.
>
> Right - it's practically inevitable that you'll need an
> anti-wraparound VACUUM to advance relfrozenxid right now. Technically
> it's possible to advance relfrozenxid in any VACUUM, but in practice
> it just never happens on a large table. You only need to get unlucky
> with one heap page, either by failing to get a cleanup lock, or (more
> likely) by setting even one single page all-visible but not all-frozen
> just once (once in any VACUUM that takes place between anti-wraparound
> VACUUMs).

But ... if I'm not mistaken, in the kind of case that Greg is
describing, relfrozenxid will be advanced exactly as often as it is
today. That's because, if VACUUM is only ever getting triggered by XID
age advancement and not by bloat, there's no opportunity for your
patch set to advance relfrozenxid any sooner than we're doing now. So
I think that people in this kind of situation will potentially be
helped or hurt by other things the patch set does, but the eager
relfrozenxid stuff won't make any difference for them.

--
Robert Haas
EDB: http://www.enterprisedb.com