You can use access method services to define and build one or more alternate indexes over a key-sequenced or entry-sequenced data set, which is called the base cluster. An alternate index provides access to records by using more than one key. The alternate index accesses records in the same way as the prime index of a key-sequenced data set. An alternate index eliminates the need to store multiple copies of the same information for different applications. The alternate index is built from all the records in a base cluster. However, it is not possible to build an alternate index from only specific records in the base cluster.
Unlike a primary key, which must be unique, the key of an alternate index can refer to more than one record in the base cluster. An alternate-key value that points to more than one record is nonunique. If the alternate key points to only one record, the pointer is unique.
Restriction: The maximum number of nonunique pointers associated with an alternate index data record cannot exceed 32 767.
Alternate indexes are not supported for linear data sets, RRDS, or reusable data sets (data sets defined with the REUSE attribute). For information about defining and building alternate indexes, see Defining Alternate Indexes.
The alternate index is a key-sequenced data set; it consists of an index component and a data component. The records in the data component contain an alternate key and one or more pointers to data in the base cluster. For an entry-sequenced base cluster, the pointers are RBA values. For a key-sequenced base cluster, the pointers are primary-key values.