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> 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. Regards, Atri
