One of two types of secondary indexes. A secondary index may be specified at table creation or at any time during the life of the table. It may consist of up to 16 columns. To get the benefit of the index, the query has to specify a value for all columns in the secondary index. A unique secondary index has two purposes: It can speed up access to a row which otherwise might require a full table scan without having to reply on the primary index, and it can be used to enforce uniqueness of a column or set of columns.