On 2 August 2015 at 13:13, Michael Paquier <michael.paqu...@gmail.com> wrote:
> Hi all, > > 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/ <http://www.2ndquadrant.com/> PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
