Example
Continuing the internet access example, a bitmap index may be logically viewed as follows:
| Identifier | HasInternet | Bitmaps | |
|---|---|---|---|
| Y | N | ||
| 1 | Yes | 1 | 0 |
| 2 | No | 0 | 1 |
| 3 | No | 0 | 1 |
| 4 | Unspecified | 0 | 0 |
| 5 | Yes | 1 | 0 |
On the left, Identifier refers to the unique number assigned to each resident, HasInternet is the data to be indexed, the content of the bitmap index is shown as two columns under the heading bitmaps. Each column in the left illustration is a bitmap in the bitmap index. In this case, there are two such bitmaps, one for "has internet" Yes and one for "has internet" No. It is easy to see that each bit in bitmap Y shows whether a particular row refers to a person who has internet access. This is the simplest form of bitmap index. Most columns will have more distinct values. For example, the sales amount is likely to have a much larger number of distinct values. Variations on the bitmap index can effectively index this data as well. We briefly review three such variations.
Note: many of the references cited here are reviewed at. For those who might be interested in experimenting with some of the ideas mentioned here, many of them are implemented in open source software such as FastBit, the Lemur Bitmap Index C++ Library, the Apache Hive Data Warehouse system and LucidDB.
Read more about this topic: Bitmap Index
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