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 UDB 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 defined, and something about which you can collect information.

The hierarchical structure is based on the relationship between the segments and the access paths that are required by the applications.

IMS uses the term database slightly differently than other DBMSs. In IMS, a database is commonly used to describe the implementation of one hierarchy, so that an application would normally access a large number of IMS databases. Compared to the relational model, an IMS segment is approximately equivalent to a table and an IMS database implements the referential integrity rules.

In This Chapter:

• "IMS Hierarchical Database Basics" on page 68

• "Basic Segment Types" on page 72

• "Sequence Fields and Access Paths" on page 73