Index-Organized Tables - Expert Indexing in Oracle Database 11g - page 77

that is built on an index-organized table must be managed differently. When creating the bitmap index
on the IOT, you must include a mapping table within the bitmap index. Again, see Chapter 3 for an
example of how to build a bitmap index on an index-organized table.
A mapping table is simply a heap-organized table that is used to store the logical ROWID values. The
mapping table is essentially an object that replaces the physical ROWID representation with a logical ROWID
representation for the rows in the table. So, within the bitmap index itself, the physical ROWID is from the
mapping table, rather than from the base table. Then the mapping table is accessed to retrieve the
logical ROWID in order to access the data from the index-organized table. See Figure 4-2 for an example of
a bitmap index with a mapping table.
Figure 4-2. A bitmap index within an index-organized table