Databases Reference
In-Depth Information
The next section discusses the makeup of bitmap indexes, when they should be used, and why they
are useful.
Figure 3-1. Sample star schema
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 (a low number of
