Internally, a bitmap and a btree indexes are very different, but functionally they are identical in that they serve to assist Oracle in retrieving rows faster than a full-table scan.
The basic differences between b-tree and bitmap indexes include:
1: Syntax differences: The bitmap index includes the "bitmap" keyword. The btree index does not say "bitmap".
2: Cardinality differences: The bitmap index is generally for columns with lots of duplicate values (low cardinality), while b-tree indexes are best for high cardinality columns.
3: Internal structure differences: The internal structures are quite different. A b-tree index has index nodes (based on data block size), it a tree form:
or--
Btree
It is made of branch nodes and leaf nodes. Branch nodes holds prefix key value along with the link to the leaf node. The leaf node in turn contains the indexed value and rowed.
Bitmap
It simply consists of bits for every single distinct value. It uses a string of bits to quickly locate rows in a table. Used to index low cardinality columns.
