Beginning Oracle Database 11g Administration From Novice to Professional

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CHAPTER 7 PHYSICAL DATABASE DESIGN
Index-Organized Tables
Ordinary Oracle tables are often referred to as heaps because the data they contain is
not sorted in any way; separate structures—indexes—are needed in order to identify
the records of interest efficiently. The index- organized table (IOT) is a single structure
that unites a table and an index for its primary key. No separate index for the primary
key is necessary, because the table itself is structured exactly as if it were an index for
the primary key; all the non- key data is stored on the leaf blocks together with the key
data. Indexes for other columns can be created in precisely the same way as indexes on
ordinary tables.
Index-organized tables have a number of advantages. The union of table and
primary key index in a single structure results in increased efficiency of retrieval operations.
Certain maintenance operations (such as the operation) can be performed
on them without invalidating all the indexes. Finally, index- organized tables can offer
dramatic performance improvements if the primary key is composed of multiple data
items and the leading item partitions the data in a natural way, for example, the store
name in a table that contains sales data. The performance improvement comes from
the physical clustering of related data that naturally results, the increased likelihood
of finding the required data in the buffer cache, and the consequent reduction in disk
activity.