Bitmap Indexes: Understanding Bitmap Indexes

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.

Continue reading

Advertisements