m_li...@yahoo.it wrote: > testinsert contains t values between '2009-08-01' and '2009-08-09', and ne_id > from 1 to 20000. But only 800 out of 20000 ne_id have to be read; there's no > need for a table scan! > I guess this is a reflection of the poor "correlation" on ne_id; but, as I > said, I don't really think ne_id is so bad correlated. > In fact, doing a "select ne_id, t from testinsert limit 100000" I can see > that data is laid out pretty much by "ne_id, t", grouped by day (that is, > same ne_id for one day, then next ne_id and so on until next day). > How is the "correlation" calculated? Can someone explain to me why, after the > procedure above,correlation is so low???
Did you run ANALYZE after the procedure above? -- Alvaro Herrera http://www.CommandPrompt.com/ The PostgreSQL Company - Command Prompt, Inc. -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
