Commit 4046e58c (dated of 2001) has introduced the following comment in vacuumlazy.c: + /* If any tuples need to be deleted, perform final vacuum cycle */ + /* XXX put a threshold on min nuber of tuples here? */ + if (vacrelstats->num_dead_tuples > 0) In short, we may want to have a reloption to decide if we do or not the last pass of VACUUM or not depending on a given number of remaining tuples. Is this still something we would like to have?
I don't think we want a new user parameter, but we should have an internal limit with a heuristic, similar to how we decide whether to truncate.
I would suggest this internal logic...
* If its a VACUUM FREEZE then index_scan_threshold = 0, i.e. always scan if needed, since the user is requesting maximum vacuum
* For emergency anti-wraparound VACUUMs we shouldn't scan indexes at all, since they aren't critical path activities at that point
* For normal VACUUMs we should scan indexes only if (num_dead_tuples * 20) > (blocks to be scanned in any one index), which allows some index bloat but not much
--
Simon Riggs http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
