A bitmap index is a special type of index that uses a series of bitstrings to represent the set of ID values that correspond to a given indexed data value. You can define a bitmap index for a field if the table's ID field is defined as a positive integer (see restrictions).
Bitmap indices have the following important features:
Bitmaps are highly compressed: bitmap indices can be significantly smaller than standard indices. This reduces disk and cache usage considerably.
Bitmaps operations are optimized for transaction processing: you can use bitmap indices within tables with no performance penalty as compared with using standard indices.
Logical operations on bitmaps (counting, AND, and OR) are optimized for high performance.
The SQL Engine includes a number of special optimizations that can take advantage of bitmap indices.
Subject to the restrictions listed below, bitmap indices operate in the same manner as standard indices. Indexed values are collated and you can index on combinations of multiple fields.