Pointers in HD databases

Before looking in detail at how HD databases are stored and processed, you need to become familiar with the pointers that are used in HD databases.

Types of pointers you can specify
In the HD access methods, segments in a database record are kept in hierarchical sequence using direct-address pointers.
Hierarchical forward pointers
With hierarchical forward (HF) pointers, each segment in a database record points to the segment that follows it in the hierarchy.
Hierarchical forward and backward pointers
With hierarchical forward and backward pointers (HF and HB), each segment in a database record points to both the segment that follows and the one that precedes it in the hierarchy (except dependent segments do not point back to root segments).
Physical child first pointers
With physical child first (PCF) pointers, each parent segment in a database record points to the first occurrence of each of its immediately dependent child segment types.
Physical child first and last pointers
With physical child first and last pointers (PCF and PCL), each parent segment in a database record points to both the first and last occurrence of its immediately dependent child segment types.
Physical twin forward pointers
With physical twin forward (PTF) pointers, each segment occurrence of a given segment type under the same parent points forward to the next segment occurrence.
Physical twin forward and backward pointers
With physical twin forward and backward (PTF and PTB) pointers, each segment occurrence of a given segment type under the same parent points both forward to the next segment occurrence and backward to the previous segment occurrence.
Mixing pointers
Because pointers are specified by segment type, the various types of pointers can be mixed within a database record. However, only hierarchical or physical, but not both, can be specified for a given segment type.
Sequence of pointers in a segment's prefix
When a segment contains more than one type of pointer, pointers are put in the segment's prefix in a specific sequence.

Parent topic: HDAM, PHDAM, HIDAM, and PHIDAM databases