A bitmap index is a special type of structure used by most high-end database management systems to optimize search and retrieval for low-variability data. Although a B-tree or B+tree is the standard index structure used for highly variable data, such as identity values, names, and street addresses, this type of index structure doesn't work especially well with data that has low variability, such as gender (M, F) or nullable (Y, N). This type of situation calls for a bitmap index, which Table A shows.

A l...