DEDB structure considerations

Several characteristics of DEDB usage affect the response time of an application program.

These characteristics include:

• Data replication
• Subset pointers
• Number of areas
• Complexity of hierarchical structure
• Complexity of DL/I calls
• Use of sharing across IMS
• Last child pointers
• Recoverability

The first three characteristics are unique to DEDBs; the last five apply generally to databases. Data replication allows up to seven data sets for an individual area. When reading from an area represented by multiple data sets, performance is not impacted, unless the CI is defective. When updating, up to seven additional writes could be required. Although the physical write is performed asynchronously to transaction processing, there could be delays caused by access paths to a variety of DASD devices.

Up to eight subset pointers allow an application program to separate the children of a parent into groups in a DEDB, with the subset pointer pointing to the start of each group. Use of such pointers can help improve performance by reducing the time needed to access segments whose position is significantly displaced in a chain of sequential dependent segments.