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Oracle Index-Organized Tables

Oracle Database Tips by Donald Burleson

About Oracle Index-Organized Tables

Oracle Index-organized tables (IOTs) are a unique style of table structure that is stored in a B-tree index structure. Besides storing the primary key values of an Oracle indexed-organized tables row, each index entry in the B-tree also stores the non-key column values. Oracle Indexed-organized tables provide faster access to table rows by the primary key or any key that is a valid prefix of the primary key. Because the non-key columns of a row are present in the B-tree leaf block itself, there is no additional block access for index blocks.

Also see: Super App to use Oracle IOT's, and Oracle Index-organized table (IOT) cluster.

Applications manipulate, select, and can update the Oracle index-organized tables just like ordinary tables, using SQL statements. The following sections show the enhancements related to Oracle that are available in the10g Database release.

LIST Partition for Oracle Index Organized tables

In the previous release of the database, you could partition an Oracle index-organized table by range or by a hash on column values. The partitioning columns had to form a subset of the primary key columns. Just like ordinary tables, local partitioned (prefixed and non-prefixed) indexes as well as global partitioned (prefixed) indexes were supported for partitioned Oracle index-organized tables. With release 10g, you will also be able to partition by the list method.

IOT - Oracle Index-organized tables - These structures remove the primary key from the table and keep them solely in the index, saving disk space. Primarily used with high-updates tables, Oracle Index Organized tables structure reduces table fragmentation. Dr. Tim Hall, author of the book "Oracle Job Scheduling" has a great discussion on the benefits of Oracle Index Organized tables:

Accessing data via the primary key is quicker as the key and the data reside in the same structure. There is no need to read an index then read the tables data in a separate structure.

Lack of duplication of the key columns in an index and table mean the total storage requirements are reduced.

Index-only tables have been around since Oracle8. If neither the Oracle  HASH or Oracle INDEX ORGANIZED options are used with the CREATE TABLE command, then a table is created as a standard hash table. If the Oracle INDEX ORGANIZED option is specified, the table is created as a B-tree-organized table identical to a standard Oracle index created on similar columns. Oracle Index-organized tables do not have rowids.

Oracle index-organized tables have the option of allowing overflow storage of values that exceed optimal index row size, as well as allowing compression to be used to reduce storage requirements. Overflow parameters can include columns and the percent threshold value to begin overflow. An index-organized table must have a primary key. Oracle index-organized tables are best suited for use with queries based on primary key values. Oracle index-organized tables can be partitioned in Oracle8i and in Oracle9i as long as they do not contain LOB or nested table types.




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