On Fri, 2008-12-12 at 16:10 +0000, Gregory Stark wrote: > Right, but increasing our sample size by a factor of 150 for a 100M > row table doesn't seem like a reasonable solution to one metric being > bogus. > > For that matter, if we do consider sampling 5% of the table we may as > well just go ahead and scan the whole table. It wouldn't take much > longer and it would actually produce good estimates.
As I said, we would only increase sample for ndistinct, not for others. At the moment we completely and significantly fail to assess ndistinct correctly on clustered data for large tables. Using block level sampling would prevent that. Right now we may as well use a random number generator. The amount of I/O could stay the same, just sample all rows on block. Lifting the sample size will help large tables. Will it be perfect? No. But I'll take "better" over "not working at all". If we are going to quote literature we should believe all the literature. We can't just listen to some people that did a few tests with sample size, but then ignore the guy that designed the MS optimizer and many others. -- Simon Riggs www.2ndQuadrant.com PostgreSQL Training, Services and Support -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
