simply using cardinality is not a good idea for deciding on bitmap indexes.
eg1: you've got 3 distinct values in colX and you want to get 'select colY where colX = 1' Sounds perfect for bitmap??? NOPE !!! Because you want 'colY', you read the bitmap but you STILL have to read bucketloads of the table...A full table scan would be far better. eg2: you're doing dml on the table...Not a good idea for bitmaps there.. Bitmaps (or several) are great if their use reduces the result set dramaticall, otherwise they can really hurt your system hth connor --- Jeremy Pulcifer <[EMAIL PROTECTED]> wrote: > I'm constantly finding, in the database that I'm > developing/supporting, > b-tree indexes on columns with low-cardinality. I > generally re-create said > index as a bitmap; however I'd like to go through > the entire db and identify > these cases proactively. Is there a way to determine > the cardinality of an > index's columns via the dictionary? Also, what > recommendations do you have > for determining the cardinality percentage that > indicates when a bitmap > should be used? 10%? 20%? > ===== Connor McDonald http://www.oracledba.co.uk http://www.oaktable.net "GIVE a man a fish and he will eat for a day. But TEACH him how to fish, and...he will sit in a boat and drink beer all day" __________________________________________________ Do You Yahoo!? Everything you'll ever need on one web page from News and Sport to Email and Music Charts http://uk.my.yahoo.com -- Please see the official ORACLE-L FAQ: http://www.orafaq.net -- Author: =?iso-8859-1?q?Connor=20McDonald?= INET: [EMAIL PROTECTED] Fat City Network Services -- 858-538-5051 http://www.fatcity.com San Diego, California -- Mailing list and web hosting services --------------------------------------------------------------------- To REMOVE yourself from this mailing list, send an E-Mail message to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in the message BODY, include a line containing: UNSUB ORACLE-L (or the name of mailing list you want to be removed from). You may also send the HELP command for other information (like subscribing).
