There is a downside to reverse key indexes, however: they need a high value for pctfree to
allow for frequent inserts, and must be rebuilt often, more often than a standard B-tree index.
Sizing Bitmap Indexes
If you create a bitmap index, Oracle will dynamically compress the bitmaps generated. The
compression of the bitmap may result in substantial storage savings. To estimate the size of a bitmap
index, estimate the size of a standard (B-tree) index on the same columns using the methods provided
in the preceding sections of this chapter. After calculating the space requirements for the B-tree
index, divide that size by 10 to determine the most likely maximum size of a bitmap index for
those columns. In general, bitmap indexes will be between 2 and 10 percent of the size of a
comparable B-tree index for a bitmap index with low cardinality. The size of the bitmap index
will depend on the variability and number of distinct values in the indexed columns; if a bitmap
index is created on a high-cardinality column, the space occupied by a bitmap index may exceed
the size of a B-tree index on the same column!
Chapter 5: Developing and Implementing Applications 149
NOTE
Bitmap indexes are only available in Oracle Enterprise Edition and
Standard Edition One.
Sizing Index-Organized Tables
An index-organized table is stored sorted by its primary key. The space requirements of an indexorganized
table closely mirror those of an index on all of the table??™s columns.
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