clear cache does not reliably remove durable temp tables
Durable (civicrm_tmp_d%) temp tables are intended for use when the table must be used in multiple transactions and therefore cannot be a true MySQL temp table. These should be cleared out when the clean cache function is triggered from the UI, scheduled jobs, or CLI -- with a few exceptions for those created within the last two days or that are tied to the civicrm_user_job table. Currently there is faulty logic that causes them to persist much longer than necessary, thus cluttering the DB.