On Aug 13, 2011, at 4:31 PM, Heikki Linnakangas wrote: >> The example is much more realistic if the query is a fetch of N latest rows >> from a table. Very common use case, and the whole relation's visibility >> statistics are completely wrong for that query. > > That is somewhat compensated by the fact that tuples that are accessed more > often are also more likely to be in cache. Fetching the heap tuple to check > visibility is very cheap when the tuple is in cache. > > I'm not sure how far that compensates it, though. I'm sure there's typically > nevertheless a fairly wide range of pages that have been modified since the > last vacuum, but not in cache anymore.
http://xkcd.org/937/ :) Could something be added to pg_stats that tracks visibility map usefulness on a per-attribute basis? Perhaps another set of stats buckets that show visibility percentages for each stats bucket? -- Jim C. Nasby, Database Architect j...@nasby.net 512.569.9461 (cell) http://jim.nasby.net -- Sent via pgsql-hackers mailing list (firstname.lastname@example.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers