At the lowest level, the links indicate the data records. The database system can move down through the levels of the tree structure to find the simple index entries that contain the location of the desired records or rows. Bitmap indices are commonly used by DBMSs to accelerate decision support queries. A bit in a bitmap is set if the corresponding RID has a given property P, e.g. a customer that lives in the Silicon Valley, but is otherwise reset. AND, OR, and NOT operations.
Bitmaps are also compact representations of densely populated sets.
By reducing the number of read operations to data, bitmapped indexes offer better response time than traditional indexing methods such as B-tree indexes. The idea behind a bitmapped index is that one bit associates a specific value for an attribute with a row. The index can identify records through their bitmap position, so bitmaps don't need pointers. In addition to performance benefits, bitmap indexes offer the added feature of requiring less storage space than their B-tree counterparts. Bitmapped indexes offer several advantages, such as reduced storage overhead and the capability to do compression operations on indexes.