Bitmap indexes are a great boon to certain kinds of application, but there is a lot of mis-information in the field about how they work, when to use them, and the side-effects. This article examines the structure of bitmap indexes, and tries to explain how some of the more commonly repeated misconceptions came into existence.
Everybody Knows …
If you did a quick survey to discover the understanding that people had of bitmap indexes, you would probably find the following comments being quoted fairly frequently:
a) When there are bitmap indexes on tables then updates will take out full table locks.
b) Bitmap indexes are good for low-cardinality columns.
c) Bitmap index scans are more efficient than tablescans even when returning a large fraction of a table.