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... > For some other databases, when you cluster on an index the table > actually *becomes an index*. This means that doing an index scan is > actually the same as a table/sequential scan, except that you can easily > find an exact place to start. Because of this, a 'clustered table' (or > an Index Organized Table as Oracle calls it) can be extremely fast for > certain operations. Ok now I know why I can only have a single clustered index per table... > In any case, remember the first rule of all performance tuning: don't. > And the second rule: if you're going to, you better have metrics to > measure your tuning with to make sure it's worth it. Yeah! EXPLAIN ANALYSE, time, systat and friends...
Thanks again, -- MaXX ---------------------------(end of broadcast)--------------------------- TIP 1: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to [EMAIL PROTECTED] so that your message can get through to the mailing list cleanly