For more information about
reverse key indexes and where to use them, refer to your Oracle documentation.
Bitmap indexes
In a standard B*-tree index, the ROWIDs are stored in the leaf blocks of the index. In
a bitmap index, each bit in the index represents a ROWID. If a particular row contains
a particular value, the bit for that row is ???turned on??? in the bitmap for that
value. A mapping function converts the bit into its corresponding ROWID. Unlike
other index types, bitmap indexes include entries for NULL values.
Basic Data Structures | 95
Youcan store a bitmap index in much less space than a standard B*-tree index if
there aren??™t many values in the index. Figure 4-2 shows an illustration of how a bitmap
index is stored. Figure 10-3 in Chapter 10 shows how a bitmap index is used in
a selection condition.
The functionality provided by bitmap indexes is especially important in data warehousing
applications in which each dimension of the warehouse contains many
repeating values, and queries typically require the interaction of several different
dimensions.
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