Retrieving segments based on the qualification of a dependent segment

Sometimes an application program needs to retrieve a segment, but only if one of the dependents of the segment meet certain qualifications.

For example, suppose that the medical clinic wants to print a monthly report of the patients who have visited the clinic during that month. If the application program that processes this request does not use a secondary index, the program has to retrieve each PATIENT segment, and then retrieve the ILLNESS segment for each PATIENT segment. The program tests the date in the ILLNESS segment to determine whether the patient has visited the clinic during the current month, and prints the patient's name if the answer is yes. The program continues retrieving PATIENT segments and ILLNESS segments until it has retrieved all the PATIENT segments.

But with a secondary index, you can make the processing of the program simpler. To do this, you index the PATIENT segment on the date field in the ILLNESS segment. When you define the PATIENT segment in the DBD, you give IMS the name of the field on which you are indexing the PATIENT segment, and the name of the segment that contains the index field. The application program can then request a PATIENT segment and qualify the request with the date in the ILLNESS segment. The PATIENT segment that is returned to the application program looks just as it would if you were not using a secondary index.

In this example, the PATIENT segment is the target segment; it is the segment that you want to retrieve. The ILLNESS segment is the source segment; it contains the information that you want to use to qualify your request for PATIENT segments. The index segment in the secondary database is the pointer segment. It points to the PATIENT segments.