🔧 OPTIMIZE TABLE ... FINAL in ClickHouse: when to use it, when to avoid it, and how merges work

If you manage a ClickHouse cluster in production, you may have hit duplicate rows or the "too many parts" error. A search result, an older blog post, or a forum thread probably pointed you to OPTIMIZE TABLE ... FINAL. You ran it, the immediate symptom vanished, and you scheduled it in a cron job.

Weeks later, ClickHouse feels slower after...