On Wed, May 25, 2011 at 5:54 PM, Tom Lane <t...@sss.pgh.pa.us> wrote: > Robert Haas <robertmh...@gmail.com> writes: >> On Wed, May 25, 2011 at 1:04 PM, Tom Lane <t...@sss.pgh.pa.us> wrote: >>> Because the problem is not specific to TOAST tables. As things >>> currently stand, we will accept the word of an ANALYZE as gospel even if >>> it scanned 1% of the table, and completely ignore the results from a >>> VACUUM even if it scanned 99% of the table. This is not sane. > >> I agree that if VACUUM scanned 99% of the table, it's probably fine to >> use its numbers. It's also fine to use the numbers from ANALYZE, >> because those pages are chosen randomly. What bothers me is the idea >> of using a small *non-random* sample, and I'm not sure that >> incorporating possibly-bogus results slowly is any better than >> incorporating them quickly. > > The above is simply fuzzy thinking. The fact that ANALYZE looked at a > random subset of pages is *no guarantee whatsoever* that its results are > highly accurate. They might be more trustworthy than VACUUM's nonrandom > sample of a similar number of pages, but it doesn't hold even a little > bit of water to claim that we should believe ANALYZE completely and > VACUUM not at all even when the latter has looked at a significantly > larger sample of pages.
I think you're arguing against a straw-man. > In any case, your line of thought doesn't help us for fixing the problem > with toast tables, because we aren't going to start doing ANALYZEs on > toast tables. Can we simply use a constant for tuple density on TOAST tables? > The bottom line here is that making use of stats we have is a lot better > than not making use of them, even if they aren't entirely trustworthy. http://dilbert.com/strips/comic/2008-05-07/ -- 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
