Bitmap index
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A bitmap index is a special kind of index that is particularly useful in data warehousing for columns that have a low number of distinct values (low cardinality). For example, Sex (M/F).
These indexes become very powerful when one is comparing different low cardinality attributes for many items. Logical operators are used to combine indexes.
Example bitmap index:
| Product Number | |||||
|---|---|---|---|---|---|
| Attribute | 1 | 2 | 3 | 4 | 5 |
| £100 | 0 | 1 | 0 | 0 | 0 |
| £200 | 1 | 0 | 0 | 0 | 0 |
| £300 | 0 | 0 | 1 | 1 | 1 |
| Garden | 1 | 0 | 0 | 0 | 1 |
| Kitchen | 0 | 1 | 1 | 1 | 0 |
| Dining Room | 0 | 1 | 0 | 1 | 0 |
Adapted from O'Connell (2005, 13.5).
In the above example, it is clear that product 4 costs £300 and that it can be used in the kitchen and dining room.
[edit] References
- O'Connell, S. (2005) Advanced Databases Course Notes, Southampton, University of Southampton.
