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: John Naylor <john.naylor@enterprisedb.com>
Cc: Jeff Davis <pgsql@j-davis.com>, Andres Freund <andres@anarazel.de>, Justin Pryzby <pryzby@telsasoft.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2022-12-13T23:07:10Z
Lists: pgsql-hackers 
On Tue, Dec 13, 2022 at 9:16 AM Peter Geoghegan <pg@bowt.ie> wrote:
> That's not the only thing we care about, though. And to the extent we
> care about it, we mostly care about the consequences of either
> freezing or not freezing eagerly. Concentration of unfrozen pages in
> one particular table is a lot more of a concern than the same number
> of heap pages being spread out across multiple tables. Those tables
> can all be independently vacuumed, and come with their own
> relfrozenxid, that can be advanced independently, and are very likely
> to be frozen as part of a vacuum that needed to happen anyway.

At the suggestion of Jeff, I wrote a Wiki page that shows motivating
examples for the patch series:

https://wiki.postgresql.org/wiki/Freezing/skipping_strategies_patch:_motivating_examples

These are all cases where VACUUM currently doesn't do the right thing
around freezing, in a way that is greatly ameliorated by the patch.
Perhaps this will help other hackers to understand the motivation
behind some of these mechanisms. There are plenty of details that only
make sense in the context of a certain kind of table, with certain
performance characteristics that the design is sensitive to, and seeks
to take advantage of in one way or another.

-- 
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