Final answer:
An efficient bitmap index has compressed storage and offers fast query performance.
Step-by-step explanation:
Two characteristics of an efficient bitmap index are compressed storage and fast query performance.
Compressed storage refers to the ability of a bitmap index to represent large amounts of data in a compact form, which reduces storage requirements and allows for faster data retrieval. This is achieved by using bitwise operations to store and manipulate the bitmap.
Fast query performance is another important characteristic of an efficient bitmap index. Bitmap indexes are designed to quickly answer queries involving multiple columns, using efficient bitwise operations that can process large amounts of data in parallel.