Horton

Re: First draft of the PG 15 release notes

Peter Geoghegan <pg@bowt.ie>

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From: Peter Geoghegan <pg@bowt.ie>
To: Bruce Momjian <bruce@momjian.us>
Cc: Noah Misch <noah@leadboat.com>, PostgreSQL-development <pgsql-hackers@postgresql.org>
Date: 2022-06-29T00:32:26Z
Lists: pgsql-hackers 
On Tue, Jun 28, 2022 at 1:35 PM Bruce Momjian <bruce@momjian.us> wrote:
> Okay, text updated, thanks.  Applied patch attached.

I have some notes on these items:

1. "Allow vacuum to be more aggressive in setting the oldest frozenxid
(Peter Geoghegan)"

2. "Add additional information to VACUUM VERBOSE and autovacuum
logging messages (Peter Geoghegan)"

The main enhancement to VACUUM for Postgres 15 was item 1, which
taught VACUUM to dynamically track the oldest remaining XID (and the
oldest remaining MXID) that will remain in the table at the end of the
same VACUUM operation. These final/oldest XID/MXID values are what we
now use to set relfrozenxid and relminmxid in pg_class. Previously we
just set relfrozenxid/relminmxid to whatever XID/MXID value was used
to determine which XIDs/MXIDs needed to be frozen. These values often
indicated more about VACUUM implementation details (like the
vacuum_freeze_min_age GUc's value) than the actual true contents of the
table at the end of the most recent VACUUM.

It might be worth explaining the shift directly in the release notes.
The new approach is simpler and makes a lot more sense -- why should
the relfrozenxid be closely tied to freezing? We don't necessarily
have to freeze any tuple to advance relfrozenxid right up to the
removable cutoff/OldestXmin used by VACUUM. For example,
anti-wraparound VACUUMs that run against static tables now set
relfrozenxid/relminmxid to VACUUM's removable cutoff/OldestXmin
directly, without freezing anything (after the first time). Same with
tables that happen to have every row deleted -- only the actual
unfrozen XIDs/MXIDs left in the table matter, and if there happen to
be none at all then we can use the same relfrozenxid as we would for
a CREATE TABLE. All depends on what the workload allows.

There will also be a real practical benefit for users that allocate a
lot of MultiXactIds: We'll now have pg_class.relminmxid values that
are much more reliable indicators of what is really going on in the
table, MultiXactId-wise. I expect that this will make it much less
likely that anti-wraparound VACUUMs will run needlessly against the
largest tables, where there probably wasn't ever one single
MultiXactId. In other words, the implementation will have more
accurate information at the level of each table, and won't .

I think that very uneven consumption of MultiXactIds at the table
level is probably common in real databases. Plus VACUUM can usually
remove a non-running MultiXact from a tuple's xmax, regardless of
whether or not the mxid happens to be before the
vacuum_multixact_freeze_min_age-based MXID cutoff -- VACUUM has
always just set xmax to InvalidXid in passing when it's possible to do so
easily. MultiXacts are inherently pretty short-lived information about
row lockers at a point in time. We don't really need to keep them
around for very long. We may now be able to truncate the two MultiXact
related SLRUs much more frequently with some workloads.

Finally, note that the new VACUUM VERBOSE output (which is now pretty
much the same as the autovacuum log output) shows when and how
relfrozenxid/relminmxid have advanced. This should make it relatively
easy to observe these effects where they exist.

Thanks

--
Peter Geoghegan

Commits

  1. Doc: last minute adjustment to the release notes

  2. Doc: more tweaking of v15 release notes.

  3. Doc: further adjust notes about pg_upgrade_output.d.

  4. Doc: add list of major features to the v15 release notes.

  5. relnotes: update item about public schema permission change

  6. relnotes: update ordered partition scan item

  7. relnotes: add Heikki to UTF8 item

  8. relnotes: improve UTF8 text item in relation to ASCII

  9. relnotes: add null logical replication item

  10. relnotes: adjust several logical replication items and FK text

  11. relnotes: mention non-exclusive backup mode was deprecated

  12. relnotes: add author to in-memory sorts item

  13. relnotes: update for non-exclusive backup mode removal

  14. relnote: improve sorting entries

  15. relnotes: adjustments from Álvaro Herrera

  16. relnotes: update foreign key partition and add sort items

  17. relnotes: more adjustments

  18. relnotes: logical replication permissions checked by subscrib.

  19. relnotes: adjustments

  20. relnotes: remove sequence replication and update 'postgres -C'

  21. relnote: extensive updates

  22. relnotes: "training" -> "trailing"

  23. Standardize references to Zstandard as <productname>

  24. pgstat: Update docs to match the shared memory stats reality.

  25. Raise a WARNING for missing publications.

  26. Skip empty transactions for logical replication.

  27. Optionally disable subscriptions on error.

  28. Allow root-owned SSL private keys in libpq, not only the backend.

  29. Use COPY FREEZE in pgbench for faster benchmark table population.