Complex sentence: A bitmap index is a special kind of database index that works well for data such as gender, which has a small number of distinct values, e.g., male and female, but many occurrences of those values.
Split into simpler sentences: A bitmap index is a special kind of database index. Bitmap indexes work well for data such as gender, which has a small number of distinct values, e.g., male and female, but many occurrences of those values.