Direct dependent segment storage

The DEDB maintains processing efficiency while supporting a hierarchical physical structure with direct dependent segment types.

A maximum of 127 segment types are supported (up to 126 direct dependent segment types, or 125 if a sequential dependent segment is present).

Direct dependent (DDEP) segment types can be efficiently retrieved hierarchically, and the user has complete online processing control over the segments. Supported processing options are insert, get, delete, and replace. With the replace function, users can alter the length of the segment. DEDB space management logic attempts to store an inserted direct dependent in the same CI that contains its root segment. If insufficient space is available in that CI, the root addressable overflow and then the independent overflow portion of the area are searched.

DDEP segments can be defined with or without a unique sequence field, and are stored in ascending key sequence.

Physical chaining of direct dependent segments consists of a physical child first (PCF) pointer in the parent for each defined dependent segment type and a physical twin forward (PTF) pointer in each dependent segment.

DEDBs allow a PCL pointer to be used. This pointer makes it possible to access the last physical child of a segment type directly from the physical parent. The INSERT rule LAST avoids the need to follow a potentially long physical child pointer chain.

Subset pointers are a means of dividing a chain of segment occurrences under the same parent into two or more groups of subsets. You can define as many as eight subset pointers for any segment type, dividing the chain into as many as nine subsets. Each subset pointer points to the start of a new subset.