On Wed, May 2, 2012 at 12:26 PM, Alvaro Herrera <alvhe...@commandprompt.com> wrote: > Excerpts from Robert Haas's message of mié may 02 08:14:36 -0400 2012: >> On Wed, May 2, 2012 at 7:16 AM, Jeroen Vermeulen <j...@xs4all.nl> wrote: >> > On 2012-05-01 22:06, Robert Haas wrote: >> >> It might also be interesting to provide a mechanism to pre-extend a >> >> relation to a certain number of blocks, though if we did that we'd >> >> have to make sure that autovac got the memo not to truncate those >> >> pages away again. >> > >> > Good point. And just to check before skipping over it, do we know that >> > autovacuum not leaving enough slack space is not a significant cause of the >> > bottlenecks in the first place? >> >> I'm not sure exactly what you mean by this: autovacuum doesn't need >> any slack space. Regular DML operations can certainly benefit from >> slack space, both within each page and overall within the relation. >> But there's no evidence that vacuum is doing too good a job cleaning >> up the mess, forcing the relation to be re-extended. Rather, the >> usual (and frequent) complaint is that vacuum is leaving way too much >> slack space - i.e. bloat. > > Hm. I see those two things as different -- to me, bloat is unremoved > dead tuples, whereas slack space would be free space that can be reused > by new tuples. Slack space is useful as it avoids relation extension; > bloat is not.
I guess I think of bloat as including both unremoved dead tuples and unwanted internal free space. If you create a giant table, delete 9 out of every 10 tuples, and vacuum, the table is still "bloated", IMV. > I wonder, though, if we should set a less-than-100 fillfactor for heap > relations. Just like default_statistic_target, it seems that the > default value should be a conservative tradeoff between two extremes. > This doesn't help extension for bulk insertions a lot, of course, but > it'd be useful for tables where HOT updates happen with some regularity. Perhaps, but in theory that should be self-correcting: the data should spread itself onto the number of pages where HOT pruning is able to prevent further relation extension. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
