Bitmap indexes can yield greatly improved performance when the data being indexed has low cardinality—that is, if there are relatively few distinct values for the indexed column. An example of a good candidate for a bitmap index would be GENDER, which would have values of “M” or “F”. A poor candidate for a bitmap index would be SALES_AMOUNT, which is likely to have a different value for almost every row.

Creating a bitmap index is similar to creating a standard index; you include the keyword BITMAP in the CREATE INDEX statement. For example, to create a bitmap index on the GENDER column of an EMPLOYEE_MASTER table, you’d specify the following statement:

CREATE BITMAP INDEX empmast_ix ON employee_master(gender);