Re: Eager page freeze criteria clarification

Hi,

On 2023-07-28 16:19:47 -0400, Robert Haas wrote:
> On Fri, Jul 28, 2023 at 3:00 PM Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
> > > Or are we trying to determine how likely
> > > the freeze record is to emit an FPI and avoid eager freezing when it
> > > isn't worth the cost?
> >
> > No, that's not something that we're doing right now (we just talked
> > about doing something like that). In 16 VACUUM just "makes the best
> > out of a bad situation" when an FPI was already required during
> > pruning. We have already "paid for the privilege" of writing some WAL
> > for the page at that point, so it's reasonable to not squander a
> > window of opportunity to avoid future FPIs in future VACUUM
> > operations, by freezing early.
>
> This doesn't make sense to me. It's true that if the pruning record
> emitted an FPI, then the freezing record probably won't need to do so
> either, unless by considerable ill-fortune the redo pointer was moved
> in the tiny window between pruning and freezing. But isn't that also
> true that if the pruning record *didn't* emit an FPI? In that case,
> the pruning record wasn't the first WAL-logged modification to the
> page during the then-current checkpoint cycle, and some earlier
> modification already did it. But in that case also the freezing won't
> need to emit a new FPI, except for the same identical case where the
> redo pointer moves at just the wrong time.
>
> Put differently, I can't see any reason to care whether pruning
> emitted an FPI or not. Either way, it's very unlikely that freezing
> needs to do so.

+many

Using FPIs having been generated during pruning as part of the logic to decide
whether to freeze makes no sense to me. We likely need some heuristic here,
but I suspect it has to look quite different than the FPI condition.

Greetings,

Andres