When to use bitmap index based on cardinality

From: <cutler_temp_at_usa.net>
Date: Fri, 05 Feb 1999 18:47:52 GMT
Message-ID: <79fecl$85p$1@nnrp1.dejanews.com>

I'm on Oracle 7.3.4.2

The Oracle Tuning documentation says this about bitmap indexes: <
Bitmap indexes can substantially improve performance of queries with the following characteristics:

The WHERE clause contains multiple predicates on low- or medium-cardinality columns.
The individual predicates on these low- or medium-cardinality columns select a large number of rows.
Bitmap indexes have been created on some or all of these low- or medium- cardinality columns.
The tables being queried contain many rows.
>

So, if I have a table of 20 million rows, and 10 columns on that table have only 50 distinct values or fewer each, and those columns are in the WHERE clause of an SQL statement, those columns are candidates for bitmap indexes.

But, what does low- or medium-cardinality really mean? If I have a column with 5000 distinct values on a 20 million row table, then the cardinality could be expressed as 5000/20,000,000 = 0.00025. This is low cardinality

compared to the number of rows, right?	Is this column a good case for bitmap
indexing?  I think it is, but I'm not sure.  Does anyone know?	I've had a

difficult time actually testing it due to the large number of rows involved. I have received different answers from Oracle support, depending on who I'm talking to.

Thanks

-----------== Posted via Deja News, The Discussion Network ==---------- http://www.dejanews.com/ Search, Read, Discuss, or Start Your Own Received on Fri Feb 05 1999 - 12:47:52 CST