Hierachical direct access datasets

The hierarchic direct databases developed to resolve some of the problems of using sequential access databases. HD databases used pointers to relate segments. Deleted segments will be physically removed from database. VSAM ESDS or OSAM data sets are used for storage. HD datasets can be stored on DASD.

Pointers are four-byte address values that provide the offset from the beginning of the data set to the segment being addressed. The segments can be tied together without the need for segments to be physically adjacent.

Segments can be inserted or deleted without the path change of other segments. HD datasets can be used the below two types of access methods

• HDAM
• HIDAM

HIDAM -

Known as Hierarchical Indexed and Direct Access Method. HIDAM combines the random processing with an index to order the database. It is the most efficient method for sequential processing with ordered database.

IMS maintains an index contains the key of the root and its address. The database is ordered. Two DBDs are required. One DBD is for the data area and one is for the index area.

The index can be processed as database. The database structure consists of two MVS datasets; the database and the index dataset. The relationships of the segments are maintained via pointers.

DASD space is immediately reusable.The space occupied by segments which are deleted is available for immediate use.

HISAM -

Known as Hierarchical Indexed Sequential Access Method.HISAM is most commonly used for low insert activity. The databases are ordered.

The database segments are stored sequentially. A parent and all its children will be sorted in physical sequence. An overflow area is maintained for the addition of segments.

The pointers are used when a dependent segment is stored in the overflow area. DASD space is not immediately reusable.A re-organization utility must be run to get the deleted segments space available for reuse.