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: Andres Freund <andres@anarazel.de>
Cc: Jeff Davis <pgsql@j-davis.com>, Matthias van de Meent <boekewurm+postgres@gmail.com>, John Naylor <john.naylor@enterprisedb.com>, Justin Pryzby <pryzby@telsasoft.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2023-01-26T01:28:48Z
Lists: pgsql-hackers 
On Wed, Jan 25, 2023 at 5:15 PM Andres Freund <andres@anarazel.de> wrote:
> However, it significantly increases the overall work when rows have a somewhat
> limited lifetime. The documented reason why vacuum_freeze_min_age exist -
> although I think it doesn't really achieve its documented goal anymore, after
> the recent changes page-level freezing changes.

Huh? vacuum_freeze_min_age hasn't done that, at all. At least not
since the visibility map went in back in 8.4:

https://wiki.postgresql.org/wiki/Freezing/skipping_strategies_patch:_motivating_examples#Today.2C_on_Postgres_HEAD_2

That's why we literally do ~100% of all freezing in aggressive mode
VACUUM with append-only or append-mostly tables.

> > VACUUM determines its freezing strategy based on the value of the new
> > vacuum_freeze_strategy_threshold GUC (or reloption) with logged tables;
> > tables that exceed the size threshold use the eager freezing strategy.
>
> I think that's not a sufficient guard at all. The size of a table doesn't say
> much about how a table is used.

Sufficient for what purpose?

> > Eager freezing is strictly more aggressive than lazy freezing.  Settings
> > like vacuum_freeze_min_age still get applied in just the same way in
> > every VACUUM, independent of the strategy in use.  The only mechanical
> > difference between eager and lazy freezing strategies is that only the
> > former applies its own additional criteria to trigger freezing pages.
>
> That's only true because vacuum_freeze_min_age being has been fairly radically
> redefined recently.

So? This part of the commit message is a simple statement of fact.

-- 
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