On Fri, May 27, 2011 at 7:36 PM, Tom Lane <t...@sss.pgh.pa.us> wrote: > Pavan Deolasee <pavan.deola...@gmail.com> writes: >> My statistical skills are limited, but wouldn't that mean that for a >> fairly well distributed write activity across a large table, if there >> are even 3-4% update/deletes, we would most likely hit a >> not-all-visible page for every 32 pages scanned ? > > Huh? With a typical table density of several dozen tuples per page, an > update ratio in that range would mean that just about every page would > have something for VACUUM to do, if the modified tuples are evenly > distributed. The case where the skip optimization has some use is where > there are large "cold" sections that have no changes at all. >
I was pretty sure that I would have done my maths wrong :-) So that means, even for far lesser update ratio, we would pretty much scan every block and vacuum many of them for a typical well distributed updates. Hmm. That means the idea of a single pass vacuum is interesting even after visibility maps. Thanks, Pavan -- Pavan Deolasee EnterpriseDB http://www.enterprisedb.com -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
