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Performance Considerations
APIs and Internal System Objects
APIs retrieve information from internal system objects. Some of the information
contains special values. For example, the list object API returns the object type as
a special value (*PGM, *LIB, and so on). However, special values may be added
in future releases. Even numeric values may have new special values. When you
code to APIs, you should assume that the format of the information returned will not
change from release to release, but the content of the information might change.
Performance Considerations
The retrieve APIs allow you to control the performance cost for information you
retrieve. The format specified for any API influences the performance cost of the
API. In general, when more information is returned, the performance is slower.
Some list APIs, such as list jobs, list spooled files, and list objects, generate the list
with minimal cost. This is why these formats do not retrieve very much information.
Some of the APIs, such as list record formats and list fields, have only one format,
because there is no additional performance cost to supply the complete information.
The retrieve APIs, such as retrieve member description and retrieve spooled file
attributes, have formats that are generally ordered from fastest performance to
slowest performance. That is, the lower numbered formats run faster but retrieve
less information, and the higher numbered formats run slower but retrieve more
information. One exception is the Retrieve Job Information (QUSRJOBI) API where
the order of the formats does not have anything to do with performance character-
istics. For more information about the performance characteristics for the
QUSRJOBI API formats, see the Retrieve Job Information (QUSRJOBI) API in the
Work Management part of the
System API Reference
book.
Chapter 2. Getting Started with APIs 2-31