[PATCH] Periodically refresh database frozen IDs during autovacuum

Hi,

Commit d7965d65, "Add rudimentary table prioritization to autovacuum"
[1], enables autovacuum to prioritize tables in urgent need of freezing,
so their per-table frozen-XID horizons can advance earlier. However, the
XID stop limit depends on the oldest database datfrozenxid, which
autovacuum normally refreshes only after a worker finishes its table
list. We have encountered a database with over 600,000 tables. Even
after the oldest tables are frozen, the database horizon can remain
stale for a long time, leaving the instance at risk of rejecting
transactions that need new XIDs.

This patch periodically refreshes the database's frozen-ID horizons
between tables. A shared per-database completion timestamp controls
periodic attempts at a fixed interval, currently set to ten minutes.
This is a tentative value for discussion, since more frequent updates
incur additional scanning overhead. I have not yet decided whether to
make the interval configurable via a GUC.

To prevent multiple workers from refreshing the same database
concurrently, periodic updates acquire the database frozen-ID lock
conditionally. If the lock is already held, another backend is handling
the database-level refresh, so the worker skips its attempt and
continues processing tables. The existing final blocking refresh is
retained as a fallback.

For testing, I constructed 1,000 tables with different frozen-XID ages,
all requiring aggressive vacuum, and enabled three autovacuum workers.
The interval was shortened to ten seconds for this test. As shown in the
attached figure, without the patch, the maximum table age fell steadily
while the database age remained flat until all tables finished at about
150 seconds. With the patch, the database age followed the declining
table age, updating roughly every ten seconds.

Feedback on the approach and interval would be welcome.

---
Regards,
Jingtang

[1] https://github.com/postgres/postgres/commit/d7965d65fc5bb2139bc51c051c11428414c65160