Purge: the VACUUM you never run
MySQL 8.4 InnoDB uses background purge to discard eligible update undo and physically remove delete-marked rows and index records. Eligibility depends on MVCC and rollback requirements. The MySQL 8.4 purge manual limits removal to "after the row is no longer required for multi-version concurrency control (MVCC) or rollback". This Documented contract supplies no demonstrated completion deadline.
Where's the transcript?
This page has no scenario of its own. History-list growth and old-row visibility demonstrate retention and visibility. They do not measure when purge finishes, guarantee eventual drainage under sustained load, or prove that undo tablespace files shrink after a reader ends.
What there is to tune
The MySQL 8.4 variable reference describes innodb_purge_threads as "The number of background threads devoted to the InnoDB purge operation." The purge system can use fewer threads than this maximum. The same reference defines innodb_purge_batch_size in undo-log pages and innodb_max_purge_lag as a threshold for delaying INSERT, UPDATE and DELETE while purge catches up. For a zero threshold it specifies "no maximum purge lag and no delay". These are tuning controls, not measurements from this site. The support assessment retains the exact parameter passages and their conditions.
Inspect old read views, write load, and the history-list trend before choosing settings. Releasing a blocking read view permits more cleanup; it does not force immediate completion. Discarding undo and truncating an undo tablespace are different operations, as the undo-tablespace manual explains. Purge also changes table/index storage by removing eligible delete-marked records; undo retention is not the only source of storage growth or read cost.
There is no PostgreSQL-style per-table VACUUM command for purge, but its configuration and workload still affect cleanup. Compare VACUUM, then continue to application patterns.
Further reading
The reconstructed support assessment records the exact manual support and execution limits for this lesson.