On Fri, Jan 10, 2014 at 11:19 PM, Atri Sharma <atri.j...@gmail.com> wrote: > > > Sent from my iPad > > On 10-Jan-2014, at 19:42, "ygnhzeus" <ygnhz...@gmail.com> wrote: > > Thanks for your reply. > So correlation is not related to the calculation of selectivity right? If I > force PostgreSQL not to optimize the join order (by setting > join_collapse_limit and from_collapse_limit to 1) , is there any other > factor that may affect the structure of execution plan regardless of the > data access method. > > 2014-01-10 > ________________________________ > ygnhzeus > ________________________________ > 发件人：Amit Langote <amitlangot...@gmail.com> > 发送时间：2014-01-10 22:00 > 主题：Re: [GENERAL] How to specify/mock the statistic data of tables in > PostgreSQL > 收件人："ygnhzeus"<ygnhz...@gmail.com> > 抄送："pgsql-general"<pgsql-general@postgresql.org> > > > > AFAIK, correlation is involved in calculation of the costs that are used for > deciding the type of access.If the correlation is low, index scan can lead > to quite some random reads, hence leading to higher costs. >
Ah, I forgot to mention this point about how planner uses correlation for access method selection. And selectivity is a function of statistical distribution of column values described in pg_statistic by histograms, most common values (with their occurrence frequencies), number of distinct values, etc. It has nothing to do with correlation. -- Amit Langote -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
