How the Database Maintains Indexes

The database automatically maintains and uses indexes after they are created. Indexes automatically reflect data changes, such as adding, updating, and deleting rows in their underlying tables, with no additional actions required by users.

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Retrieval performance of indexed data remains almost constant, even as rows are inserted. However, the presence of many indexes on a table degrades DML performance because the database must also update the indexes.

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Index Storage

Oracle Database stores index data in an index segment.

Space available for index data in a data block is the data block size minus block overhead, entry overhead, rowid, and one length byte for each value indexed.

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The tablespace of an index segment is either the default tablespace of the owner or a tablespace specifically named in the CREATE INDEX statement. For ease of administration you can store an index in a separate tablespace from its table. For example, you may choose not to back up tablespaces containing only indexes, which can be rebuilt, and so decrease the time and storage required for backups.

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