Alternate Index

AIX

Enhanced Definition

An Alternate Index (AIX) is a VSAM component that provides an alternative access path to records in a VSAM Key -Sequenced Data Set (KSDS) or Entry -Sequenced Data Set (ESDS) based on a key other than the primary key. It allows applications to retrieve records using a secondary key, which can be unique or non-unique, offering flexible data access.

Key Characteristics

- Secondary Key Access: Enables applications to access records in a base cluster (KSDS or ESDS) using a key field different from the primary key, such as a customer name or product code.
- Non-Unique Keys: Supports alternate keys that can be non-unique, meaning multiple records in the base cluster can share the same alternate key value.
- Path Component: An AIX is typically accessed via a PATH entry, which logically connects the alternate index to its base cluster, simplifying application programming.
- Automatic Maintenance: VSAM automatically maintains the alternate index whenever records in the base cluster are added, deleted, or updated, ensuring consistency between the base cluster and its AIX.
- Structure: An AIX itself is a VSAM KSDS, where its key is the alternate key from the base cluster, and its data component contains the primary key(s) of the corresponding base cluster records.
- Overhead: While providing flexibility, the automatic maintenance of AIXs can introduce I/O and CPU overhead during base cluster update operations.

Use Cases

- Online Transaction Processing (OLTP): Allowing CICS transactions to quickly retrieve customer records using an alternate key like CUSTOMER-NAME when the primary CUSTOMER-ID is unknown.
- Batch Reporting: Generating reports sorted or filtered by a field other than the primary key, such as listing all employees in a specific DEPARTMENT-CODE.
- Data Integration: Providing different applications with their preferred access method to the same base data, each using a relevant secondary key.
- Ad-hoc Queries: Facilitating flexible data retrieval for ad-hoc queries where the primary key is not the most convenient lookup criterion.

Related Concepts

An Alternate Index is fundamentally tied to VSAM (Virtual Storage Access Method), specifically enhancing access to KSDS and ESDS base clusters. It is defined using JCL commands like DEFINE AIX and DEFINE PATH within a IDCAMS utility job. Conceptually, it serves a similar purpose to secondary indexes in DB2 or IMS, providing an additional indexed access path to data records, but is managed directly by VSAM.

Best Practices:

Strategic Implementation: Define alternate indexes only when a clear business need for secondary key access exists, as each AIX adds overhead to base cluster maintenance.
Path Utilization: Always access the base cluster through the PATH associated with the AIX; this ensures proper VSAM management and simplifies application logic.
Performance Monitoring: Regularly monitor the performance impact of AIX maintenance, especially for highly volatile base clusters, and consider tuning or reorganization if performance degrades.
Rebuild and Verify: After significant data changes, recovery operations, or when performance issues arise, use IDCAMS BLDINDEX to rebuild the AIX and ensure its consistency with the base cluster.
Space Allocation: Carefully estimate and allocate sufficient primary and secondary space for the AIX, considering the number of records, key length, and potential for duplicate alternate keys.