What is a bitmap index?

What is a bitmap index?

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A bitmap index is used for data warehouse applications, where there are large amounts of data. Queries are more frequently issued than data updates. Hence, bitmap index proves more efficient than conventional indexes in retrieving data. A bitmap index is built on columns with low cardinality, unlike the B-tree index that is built on columns with high cardinality. The bitmap index uses a bitmap to indicate that a row with the required key value exists. It does so by setting the bit at the corresponding column of the bitmap to 1. For example, consider that the bitmap index is built on a column that contains three distinct city names, Las Vegas, New York, and Paris. A conventional index employing the row-id value would look such as the one below:

City	Row-id
New York	SS1025AA
Las Vegas	SS1025AB
New York	SS1025AC
Paris	SS1025AD
New York	SS1025AE

But, a bitmap index would be such as:

City	Bitmap
New York	10101
Las Vegas	01000
Paris	00010

The bitmap uses less storage space than conventional indexes such as the B-Tree index. It is also faster than other indexes because it uses binary mapping of rows, for which the retrieval time is comparatively low.

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