Physical Databases

A Physical Database is a DBD Definition
A Physical Database is a DBD Definition
In DL/I, a
physical database
is a DBD definition that specifies the allowable segments, segment fields, and segment relationships for an actual database as stored on disk. Such a definition is known as a
physical DBD
. The term "physical" in this context is somewhat misleading because the DBD serves as the top-level logical definition (or template) for the database. All of the DBD definitions examined thus far are examples of physical databases, even though they define logical as well as hierarchical relationships.
What is a Physical DBD
A physical DBD maps the definition of segments and their hierarchical relationships to physical storage. The sequence in which the segments are defined in the DBD determines how their occurrences will be stored on disk. The hierarchical relationships determine the access path that must be navigated to reach a specific segment occurrence.
A Physical DBD Specifies an Access Method
In addition to defining segments and their relationships, a physical DBD specifies the physical data organization to be used and the corresponding
access method
. DL/I provides four physical access methods: HDAM, HISAM, HIDAM, and HSAM. The choice of access method is the responsibility of the database designer and depends on the contents of the database and the transaction load requirements. The choice of access method is described in more detail under Physical Access Methods.
Sample DBD Statement
Physical DBDs can be easily identified because they specify one of the four access methods for the ACCESS parameter in the DBD statement. For example, the DBD for PHYSDB1 in Figure 9 is a physical DBD. The DBD statement is as follows:
                DBD NAME = PHYSDB1,ACCESS=HDAM
The diagram below shows the physical database (hierarchy) derived from the DBD source statements in Figure 7.
IDMSDB--Physical Databases
Figure 13. Sample physical databases