Re: Cardinality of indexed columns... how to discover? What is the ma

From: Connor McDonald <hamcdc_at_yahoo.co.uk>
Date: Fri, 20 Dec 2002 13:29:35 -0800
Message-ID: <F001.0052070F.20021220132935@fatcity.com>

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_at_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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Received on Fri Dec 20 2002 - 15:29:35 CST