Indexed

Enhanced Definition

In the mainframe context, "indexed" refers to a method of organizing data where a separate structure, known as an index, is created to facilitate rapid direct access to records or data based on one or more key fields. This organization significantly improves retrieval performance compared to sequential scanning, particularly for large datasets.

Key Characteristics

- Direct Access Capability: Allows for efficient retrieval of specific records without reading through an entire dataset, crucial for online transaction processing (OLTP) and random access applications.
- Key-Based Retrieval: Data is accessed by specifying a key value (e.g., customer ID, employee number) which the index maps to the physical location of the corresponding record.
- Overhead: Maintaining an index incurs overhead in terms of storage space for the index structure itself and processing time for updates (insertions, deletions, modifications) to keep the index synchronized with the data.
- Data Integrity: The index must accurately reflect the state of the data it points to; inconsistencies can lead to data access errors or incorrect results.
- Common Implementations: Widely used in VSAM Key-Sequenced Data Sets (KSDS), DB2 table indexes, and IMS database secondary indexes.

Use Cases

- VSAM KSDS Files: Storing customer master records where direct access by customer ID is frequently required for inquiries or updates in COBOL batch or CICS online transactions.
- DB2 Database Tables: Creating indexes on columns frequently used in WHERE clauses (e.g., ORDER_ID in an ORDERS table) to accelerate query execution and improve application response times.
- IMS Database Secondary Indexing: Providing alternative access paths to segments within an IMS database, allowing applications to retrieve data based on fields other than the primary key (root segment key).
- COBOL Indexed Files: Defining ORGANIZATION IS INDEXED in a COBOL program's SELECT statement to process files that support random, sequential, or dynamic access modes using record keys.

Related Concepts

"Indexed" is fundamental to various mainframe data management systems. It is intrinsically linked to VSAM KSDS, where the index component maps key ranges to control intervals. In DB2, indexes are critical for optimizing SQL query performance, often working in conjunction with the DB2 Optimizer to determine the most efficient access path. For IMS, secondary indexes provide crucial flexibility for accessing hierarchical data beyond the primary key. COBOL programs leverage indexed file organizations for efficient direct and sequential I/O operations, managing FILE STATUS codes for indexed file operations.

Best Practices:

Strategic Index Design: For DB2, create indexes on columns frequently used in WHERE, JOIN, ORDER BY, or GROUP BY clauses, prioritizing high-selectivity columns to minimize I/O.
Regular Reorganization: Periodically reorganize indexed datasets (e.g., VSAM KSDS, DB2 indexes) to reclaim fragmented space, improve physical contiguity, and maintain optimal access performance.
Monitor Index Usage: For DB2, use tools like EXPLAIN and monitor system catalog tables (SYSIBM.SYSINDEXES) to identify unused or inefficient indexes that can be dropped or modified.
Minimize Index Overhead: Avoid over-indexing, as each index adds overhead to data modification operations (inserts, updates, deletes). Balance read performance gains against write performance costs.
Backup and Recovery: Ensure that indexed data structures and their corresponding indexes are included in regular backup and recovery procedures to maintain data integrity and availability.