After an alternate index is defined, you need to establish the relationship between an alternate index and its base cluster, using the access method services command, DEFINE PATH. You must name the path and can also give it a password. The path name refers to the base cluster/alternate index pair. When you access the data set through the path, you must specify the path name in the DSNAME parameter in the JCL.

When your program opens a path for processing, both the alternate index and its base cluster are opened. When data in a key-sequenced base cluster is read or written using the path's alternate index, keyed processing is used. RBA processing is permitted only for reading or writing an entry-sequenced data set's base cluster.

Related reading: See z/OS DFSMS Access Method Services Commands for information about using the DEFINE PATH command.