Re: Vacuum statistics

Hi, all!

Attached is a reworked version of the patch set. The main change is that the
statistics are no longer exposed through a contrib extension; they are now
built-in system views - pg_stat_vacuum_tables, pg_stat_vacuum_indexes and
pg_stat_vacuum_database - backed by pg_stat_get_vacuum_*() functions and 
stored
in the cumulative statistics system under a dedicated stats kind
(PGSTAT_KIND_VACUUM_RELATION). Collection is controlled by a single
track_vacuum_statistics GUC; the per-relation tracking list, the reset
function and the shared-memory-size helper from the extension version 
are gone.

The series is also split into smaller, self-contained commits - one per 
metric
category (core heap/tuple counters, missed dead tuples/pages, visibility-map
transitions, the buffer/WAL/timing sampling machinery, per-relation buffers,
timing and the wraparound failsafe, and WAL) - with the documentation and
tests growing inside each commit.

Other changes:

  * index statistics are accumulated across the bulkdelete and cleanup
    passes and reported once per index, in both the serial and parallel
    paths;
  * index_vacuum_count is no longer exposed;
  * the regression coverage is now an in-core vacuum_stats test with
    deterministic checks and dedicated scenarios for the non-trivial
    paths (truncation, freeze, cost delay, WAL full-page images, index
    page deletion), an isolation test for recently_dead/missed_dead
    tuples, and a xid_wraparound TAP test for the failsafe counter.

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Best regards,
Alena Rybakina
Yandex Cloud