To retrieve records for a request, a smaller subtree structure is constructed from the structure defined by your Master File. The subtree consists of segments that contain fields named explicitly in the request and those named implicitly by virtual fields or calculated values. The subtree also includes any segments needed to connect these segments.

For example, if a TABLE request needs fields from segments D, G, and I, the subtree shown in the following diagram is constructed:

Segments C and B do not contain fields needed for the request, but they are included in the subtree to connect segment I with segments D and G. Since the segments are descendants of segment B, the entry or root segment of the subtree is segment B. The Master File root segment A is not included because its fields are not referenced. The records corresponding to segment B can be obtained independently of A. However, if segment B were an OCCURS segment, the IDMS calls would be issued for segment A to obtain information for segment B.