On Wed, Sep 27, 2023 at 1:03 PM Andres Freund <andres@anarazel.de> wrote:
> I suspect that medium term the better approach would be to be much more
> aggressive about setting all-visible, including as part of page-level
> visibility checks, and to deal with the concern of vacuum not processing such
> pages soon by not just looking at unmarked pages, but also a portion of the
> all-visible-but-not-frozen pages (the more all-visible-but-not-frozen pages
> there are, the more of them each vacuum should process). I think all-visible
> is too important for index only scans, for us to be able to remove it, or
> delay setting it until freezing makes sense.
>
> My confidence in my gut feeling isn't all that high here, though.
I think that this is a bad idea. I see two main problems with
"splitting the difference" like this:
1. If we randomly scan some all-visible pages in non-aggressive
VACUUMs, then we're sure to get FPIs in order to be able to freeze.
As a general rule, I think that we're better of gambling against
future FPIs, and then pulling back if we go too far. The fact that we
went one VACUUM operation without the workload unsetting an
all-visible page isn't that much of a signal about what might happen
to that page.
2. Large tables (i.e. the tables where it really matters) just don't
have that many VACUUM operations, relative to everything else. Who
says we'll get more than one opportunity per page with these tables,
even with this behavior of scanning all-visible pages in
non-aggressive VACUUMs?
Big append-only tables simply won't get the opportunity to catch up in
the next non-aggressive VACUUM if there simply isn't one.
--
Peter Geoghegan