Unique and Nonunique Indexes

Indexes can be unique or nonunique. Unique indexes guarantee that no two rows of a table have duplicate values in the key column or columns.

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For example, your application may require that no two employees have the same employee ID. In a unique index, one rowid exists for each data value. The data in the leaf blocks is sorted only by key.

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Nonunique indexes permit duplicates values in the indexed column or columns. For example, the first_name column of the employees table may contain multiple Mike values. For a nonunique index, the rowid is included in the key in sorted order, so nonunique indexes are sorted by the index key and rowid (ascending).

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Oracle Database does not index table rows in which all key columns are null, except for bitmap indexes or when the cluster key column value is null.

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