Hi, On 25.1.2015 00:33, AlexK987 wrote: > The documentation states that "The extent of analysis can be > controlled by adjusting the default_statistics_target configuration > variable". It looks like I can tell Postgres to create more > histograms with more bins, and more distinct values. This implicitly > means that Postgres will use a larger random subset to calculate > statistics. > > However, this is not what I want. My data may be quite skewed, and I > want full control over the size of the sample. I want to explicitly > tell Postgres to analyze the whole table. How can I accomplish that? I don't think there's an official way to do that - at least I can't think of one. The only thing you can do is increasing statistics target (either globally by setting default_statistics_target, or per column using ALTER TABLE ... SET STATISTICS). As you noticed, this however controls two things - sample size and how detailed the statistics (MCV list / histogram) will be. The statistics target is used as upper bound for number of MCV items / histogram bins, and the number of sampled rows is (300 * statistics_target). With default_statistics_target = 10000 (which si the max allowed value since 9.0), this produces very detailed stats and uses sample of ~3M rows. It's a bit more complicated though, because there's an algorithm that decides how many MCV items / histogram buckets to actually create, based on the data. So you may not get more detailed stats, even when using larger sample. That being said, I really doubt increasing the statistics target above 10000 (or even sampling the whole table) will help you in practice. Might be worth showing an example of a bad estimate with your data, or maybe a test case to play with. -- Tomas Vondra http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
