Direct access

The advantage of direct access processing is that you can get good results for both direct and sequential processing. Direct access means that by using a randomizing routine or an index, IMS can find any database record that you want, regardless of the sequence of database records in the database.

IMS full function has four direct access methods.

• HDAM and PHDAM process data directly by using a randomizing routine to store and locate root segments.
• HIDAM and PHIDAM use an index to help them provide direct processing of root segments.

The direct access methods use pointers to maintain the hierarchic relationships between segments of a database record. By following pointers, IMS can access a path of segments without passing through all the segments in the preceding paths.

Some of the requirements that direct access satisfies are:

• Fast direct processing of roots using an index or a randomizing routine
• Sequential processing of database records with HIDAM and PHIDAM using the index
• Fast access to a path of segments using pointers

In addition, when you delete data from a direct-access database, the new space is available almost immediately. This gives you efficient space utilization; therefore, reorganization of the database is often unnecessary. Direct access methods internally maintain their own pointers and addresses.

A disadvantage of direct access is that you have a larger IMS overhead because of the pointers. But if direct access fulfills your data access requirements, it is more efficient than using a sequential access method.