Bitmap indexes are widely used for indexing warehouse data.
A bitmap index (BI) allows fast access to tuples based on values of attributes. Bitmap
indexes consume only a fraction of the size of the indexed data and provide dramatic
performance gains. Boolean operations such as AND, OR and NOT are extremely
fast for bitmap vectors, also called bitmaps or bit-vectors (O??™Neil & Quass, 1997).
Bitmaps indicate whether an attribute in a tuple is equal to, greater than or less than
(depending upon the type of BI) a specific value or not. The length of a bit-vector
is equal to the cardinality of the indexed table. The position of a bit in a bit-vector
denotes the position of a tuple in the table. For example, a simple bitmap index
(SBI) on an attribute status, with domain {backorder, shipped}, results in two bitmap
vectors, say Bb and Bs. For Bb, the bit is set to 1 if the corresponding tuple has the
value ???backorder??? for the attribute status, otherwise the bit is set to 0. Similarly for
Bs, the bit is set to 1 if the associated tuple has the value ???shipped??? for the attribute
status, otherwise the bit is set to 0. For another attribute, say product-category having
values from 1-5, there is a bitmap vector corresponding to each of the five values,
say B1-B5. Tuples that have product-category value as 1 have the bit corresponding
to bit-vector B1 set; the rest of the bits for that tuple are 0.
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