On Wed, Nov 02, 2005 at 10:55:36PM +0100, MaXX wrote: > Jim C. Nasby wrote: > > On Wed, Nov 02, 2005 at 08:50:45PM +0100, MaXX wrote: > [...] > >> In simple words: > >> Clustered indexes are like the alphabetical index in a book, where term > >> are randomly distibuted in the book and regular indexes are more like the > >> table of content... > >> Right? > > You have that backwards. The TOC matches the ordering of the book > > (table). Think of it as the book is clustered on the TOC. Stuff from the > > index appears all over; it's not clustered. > > Keep in mind that for PostgreSQL it's simply a matter of correlation. > > You can actually see correlation in one of system views. The higher the > > correlation between an index and the table, the more efficient index > > scans will be. > pgAdmin shows a correlation value in the statistics panel when I click on a > column... Not sure if it is the right one as all column have it, look more > correlation between values in the column... > Still have a *lot* of things to learn... That's because unfortunately PostgreSQL only keeps statistics on individual columns. There's no stats kept on multi-column indexes; the best the planner can do is use the stats for the first column. -- Jim C. Nasby, Sr. Engineering Consultant jnasby@xxxxxxxxxxxxx Pervasive Software http://pervasive.com work: 512-231-6117 vcard: http://jim.nasby.net/pervasive.vcf cell: 512-569-9461 ---------------------------(end of broadcast)--------------------------- TIP 6: explain analyze is your friend
