Database Bitmap index

I don't understad why bitmasps indexes are useful:

Ident. Name Gender  Bitmaps
                    F     M
1   Ann     Female  1   0
2   John    Male    0   1
3   Jacob   Male    0   1
4   Pieter  Unsp.   0   0
5   Elise   Female  1       0

If query need to find all person with some Gender - it is clear. But when e.g. need to find all that name starts from开发者_如何学Python "J" ?

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Bitmaps are generally useful only for columns like Gender where the number of distinct values is fairly small. You would not use a bitmap index on names. They are also more useful in data warehousing than in OLTP databases due to the higher cost of maintaining bitmap indexes.

One advantage of bitmap indexes is that a number of them can be ANDed and ORed together to answer queries very efficiently.