Re: Bitmap Index, when use it?

On Jan 28, 5:50 am, Cassiano <cassianorol..._at_gmail.com> wrote:
> Hello,
>
> I've a table wich 6 milions of rows, and this table have one column
> "TYPE" char(2).
> This column is used to descript the type of the row.
>
> This column have 35 distict values and is frequently used in "where"
> clause.
>
> I think this is a good column to create an Bitmap index? It's true? or
> 35 distict values is so much to bitmap indexes?
>
> Thanks for help.
>
> Cassiano Roloff
> DBA Oracle - Porto Alegre/RS/Brazil

http://jonathanlewis.wordpress.com/2006/11/29/bitmap-indexes/ If you don't like .doc files: http://www.dbazine.com/oracle/or-articles/jlewis3

http://www.orafaq.com/usenet/comp.databases.oracle.server/2007/03/16/0826.htm

Note David's wording, "a good candidate," you must test for your particular situation. It could happen that things might be slower if the bitmap is used followed by what is needed for the rest of the query, while if it is not used a skip scan or even a table scan might be better. It could be different depending on version and how complicated the queries are and how well you tell the optimizer about everything else. Often the real determinant is how other queries access the db and the management of the bitmap indices. If your system is OLTP rather than DW, for example. See the docs: http://www.oracle.com/pls/db102/search?word=bitmap+index&partno=

If you aren't familiar with skip scans, check the docs.

jg

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