ALCINDEX - Alternate Index Cluster

Key Characteristics

• • Secondary Access Path: It provides an alternative way to locate records in a VSAM KSDS (Key-Sequenced Data Set) or ESDS (Entry-Sequenced Data Set) base cluster, using a key different from the base cluster's primary key.
  • KSDS Structure: An Alternate Index itself is always structured like a KSDS, consisting of an index component and a data component, regardless of whether the base cluster is KSDS or ESDS.
  • Pointer Records: The data component of an Alternate Index contains records where the alternate key is paired with the primary key (for KSDS base) or the RBA (for ESDS base) of the corresponding record in the base cluster.
  • Non-Unique Keys: An Alternate Index can be defined with non-unique keys, meaning multiple records in the base cluster can share the same alternate key value. In such cases, the AIX data component will contain multiple entries for that alternate key, each pointing to a different base cluster record.
  • IDCAMS Utility: It is defined using the DEFINE AIX command and populated using the BLDINDEX command of the IDCAMS (Access Method Services) utility.
  • Update Attribute: Can be defined with the UPDATE attribute, allowing VSAM to automatically update the alternate index whenever the base cluster is modified (insertions, deletions, updates that change the alternate key field).

Use Cases

• • Flexible Data Retrieval: Accessing a customer master file by CUSTOMER_NAME when the primary key is CUSTOMER_ID, enabling queries and reports based on different criteria.
  • Reporting and Analytics: Generating reports that require data sorted or selected based on a field that is not the primary key, such as listing all employees by DEPARTMENT_CODE.
  • Batch Processing: Efficiently processing records in a specific order for a batch job, without needing to sort the entire base cluster first.
  • Online Transaction Processing (CICS/IMS): Providing quick access to records in online applications when users search by alternative identifiers (e.g., finding an order by ORDER_DATE instead of ORDER_NUMBER).

Related Concepts