Cardinality:

The benefits of using bitmap indexes are greatest for low cardinality columns: which is, columns in which the number of distinct values is small compared to the number of rows in the table. If the values in a column are frequent more than a hundred times then the column is a candidate for a bitmap index. An Even columns with a lower number of repetitions (and therefore higher cardinality), can be candidates if they tend to be included in complex conditions in the WHERE clauses of queries.

For instance, in a table with a column and one million rows with 10,000 distinct values is a candidate for a bitmap index. The bitmap index on this column can out-perform a B*-tree index and particularly when this column is frequent queried in conjunction with other columns.

The B*-tree indexes are most effectual for high-cardinality data: which is, data with several possible values, like as CUSTOMER_NAME/PHONE_NUMBER. A regular B*-tree index can be many times larger than the indexed data. Used properly, bitmap indexes can be significantly smaller than a corresponding B*-tree index.

In ad hoc queries and same conditions, bitmap indexes can dramatically improve query performance.  AND and OR conditions in the WHERE clause of a query can be rapidly resolved through performing the corresponding boolean operations straightly on the bitmaps before converting the resulting bitmap to rowids. If the concluded number of rows is small and the query can be answered very rapidly without resorting to a full table scan of the table.