Bitmap Indexes allow versatile querying of Fact tables in a Data Warehouse.
The Cost Based Optimizer will decide whether a Bitmap Index access path is appropriate. If Oracle believes that the Bitmap Index access path will return a significant proportion of the table, or if it believes that there is a better conventional (b-tree) index acces path, then it will refuse to use the bitmap indexes.
To force Oracle to use the Bitmap Indexes, use the INDEX_COMBINE hint. This hint can also be used to encourage Oracle to include a particular bitmap index in its access path, as it may deliberately exclude indexes that it believes will provide little or no benefit.