Re: Bitmap Index

From: Jonathan Lewis <jonathan_at_jlcomp.demon.co.uk>
Date: 1998/02/19
Message-ID: <01bd3d88$467b3c50$294b989e@WORKSTATION>#1/1

An important point to note with Bitmap indexes is that they are only useful in combinations.

i.e. a query on GENDER alone ought to ignore a bitmap index and use a full tablescan, but a query on:

	gender			(2 values)
	colour of eyes		(several values)
	colour of hair		(several values)
	number of missing teeth	(33 values)
	height to nearest cm	(ca. 250 values)

could combine several 'very poor' indexes to produce a very small list of hits on the table.

Bitmap indexes should not really be compared with b-tree indexes - their mode of use is not the same. Received on Thu Feb 19 1998 - 00:00:00 CST