Understanding Bitmap Indexes
Bitmap indexes differ greatly from the traditional B-tree index discussed in Chapter 2. A bitmap index is composed of several strings of bits. Each bit string represents one of the valid distinct values in the underlying column. Each bit is either on or off, indicating whether the value applies to a given row. Table 3-1 shows an example involving the GENDER column from the EMPLOYEES table. Notice how the index has a bit string for each value: male and female. Individual bits in each string indicate whether a given row has the value of male or female.
Bitmap indexes can be created very quickly and can end up being very small in relation to a comparable B-tree index. Bitmap indexes are built for columns with low cardinality ...
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