Message-ID: <F001.0052070F.20021220132935@fatcity.com>
Date: Fri, 20 Dec 2002 13:29:35 -0800
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From: =?iso-8859-1?q?Connor=20McDonald?= <hamcdc@yahoo.co.uk>
Subject: Re: Cardinality of indexed columns... how to discover? What is the ma
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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 <Jeremy.Pulcifer@kadiri.com>
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"

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Author: =?iso-8859-1?q?Connor=20McDonald?=
  INET: hamcdc@yahoo.co.uk

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