Horton

Re: New strategies for freezing, advancing relfrozenxid early

Peter Geoghegan <pg@bowt.ie>

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From: Peter Geoghegan <pg@bowt.ie>
To: Jeff Davis <pgsql@j-davis.com>
Cc: Matthias van de Meent <boekewurm+postgres@gmail.com>, John Naylor <john.naylor@enterprisedb.com>, Andres Freund <andres@anarazel.de>, Justin Pryzby <pryzby@telsasoft.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2022-12-31T00:58:12Z
Lists: pgsql-hackers 
On Fri, Dec 30, 2022 at 1:12 PM Peter Geoghegan <pg@bowt.ie> wrote:
> > "Nominal freezing" is happening when there are no freeze plans at all.
> > I get that it's to manage control flow so that the right thing happens
> > later. But I think it should be defined in terms of what state the page
> > is in so that we know that following a given path is valid. Defining
> > "nominal freezing" as a case where there are no freeze plans is just
> > confusing to me.
>
> What would you prefer? The state that the page is in is not something
> that I want to draw much attention to, because it's confusing in a way
> that mostly isn't worth talking about.

I probably should have addressed what you said more directly. Here goes:

Following the path of freezing a page is *always* valid, by
definition. Including when there are zero freeze plans to execute, or
even zero tuples to examine in the first place -- we'll at least be
able to perform nominal freezing, no matter what. OTOH, following the
"no freeze" path is permissible whenever the freeze_required flag
hasn't been set during any call to heap_prepare_freeze_tuple(). It is
never actually mandatory for lazy_scan_prune() to *not* freeze.

It's a bit like how a simple point can be understood as a degenerate
circle of radius 0. It's an abstract definition, which is just a tool
for describing things precisely -- hopefully a useful tool. I welcome
the opportunity to be able to describe things in a way that is clearer
or more useful, in whatever way. But it's not like I haven't already
put in significant effort to this exact question of what "freezing the
page" really means to lazy_scan_prune(). Naming things is hard.

-- 
Peter Geoghegan

Commits

  1. Revert "Add eager and lazy freezing strategies to VACUUM."

  2. Add eager and lazy freezing strategies to VACUUM.

  3. Refine the definition of page-level freezing.

  4. Avoid special XID snapshotConflictHorizon values.

  5. Add page-level freezing to VACUUM.

  6. Remove overzealous MultiXact freeze assertion.

  7. Refactor how VACUUM passes around its XID cutoffs.

  8. Deduplicate freeze plans in freeze WAL records.

  9. Set relfrozenxid to oldest extant XID seen by VACUUM.

  10. Only skip pages marked as clean in the visibility map, if the last 32

  11. Add vacuum_freeze_table_age GUC option, to control when VACUUM should