SSA guidelines

Using SSAs can simplify your programming, because the more information you can give IMS to do the searching for you, the less program logic you need to analyze and compare segments in your program.

Using SSAs does not necessarily reduce system overhead, such as internal logic and I/Os, required to obtain a specific segment. To locate a particular segment without using SSAs, you can issue DL/I calls and include program logic to examine key fields until you find the segment you want. By using SSAs in your DL/I calls, you can reduce the number of DL/I calls that are issued and the program logic needed to examine key fields. When you use SSAs, IMS does this work for you.

Recommendations:

Use qualified calls with qualified SSAs whenever possible. SSAs act as filters, returning only the segments your program requires. This reduces the number of calls your program makes, which provides better performance. It also provides better documentation of your program. Qualified SSAs are particularly useful when adding segments with insert calls. They ensure that the segments are inserted where you want them to go.
For the root segment, specify the key field and an equal relational operator, if possible. Using a key field with an equal-to, equal-to-or-greater-than, or greater-than operator lets IMS go directly to the root segment.
For dependent segments, it is desirable to use the key field in the SSA, although it is not as important as at the root level. Using the key field and an equal-to operator lets IMS stop the search at that level when a higher key value is encountered. Otherwise IMS must search through all occurrences of the segment type under its established parent in order to determine whether a particular segment exists.
If you often must search for a segment using a field other than the key field, consider putting a secondary index on the field.

For example, suppose you want to find the record for a patient by the name of Ellen Carter. As a reminder, the patient segment in the examples contains three fields: the patient number, which is the key field; the patient name; and the patient address. The fact that patient number is the key field means that IMS stores the patient segments in order of their patient numbers. The best way to get the record for Ellen Carter is to supply her patient number in the SSA. If her number is 09000, your program uses this call and SSA:

GUbbbbbbPATIENTb(PATNObbb=b09000)

If your program supplies an invalid number, or if someone has deleted Ellen Carter's record from the database, IMS does not need to search through all the PATIENT occurrences to determine that the segment does not exist.

However, if your program does not have the number and must give the name instead, IMS must search through all the patient segments and read each patient name field until it finds Ellen Carter or until it reaches the end of the patient segments.