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An Introduction to IMS: Your Complete Guide to IBM Information Management System by Steve Nathan, Sandy Sherrill, Rick Long, Moira McFadden Lanyi, Margaret Wilson, Kenny Blackman, John Butterweck, Diane Goff, Barbara Klein

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Chapter 7. Overview of the IMS Hierarchical Database Model

IMS uses a hierarchical database model as the basic method of storing data. Unlike the relational model used by DB2 for z/OS, which was the result of theoretical work,1 the hierarchical model was arrived at as a pragmatic way of storing and retrieving data quickly while using as few computer resources as possible.

1 “A Relational Model of Data for Large Shared Data Banks,” paper by Dr. E. F. Codd that was published in Communications of the Association for Computing Machinery, June, 1970.

In the hierarchical model, individual data types are implemented as segments in a hierarchical structure. A segment is the smallest amount of data that can be transferred by one IMS operation, is uniquely ...

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