Guide on Creating VSAM Alternate Index using IDCAMS

Here is an example that shows how to create an VSAM alternate index using the IDCAMS utility. Full details are available here.

VSAM (Virtual Storage Access Method)

It is a file management system used in IBM mainframe operating systems. It provides efficient access to large volumes of data, both sequentially and randomly. VSAM organizes data into various file structures, such as Key Sequenced Data Set (KSDS), Entry Sequenced Data Set (ESDS), and Relative Record Data Set (RRDS).

Features of VSAM

One of the features of VSAM is the ability to create alternate indexes. An alternate index is an additional index built on top of a base cluster, allowing access to the data beyond the defined key. It can be defined for both KSDS and ESDS files. Alternate indexes provide benefits like reducing data redundancy and enabling both sequential and random access to datasets.

However, there are some disadvantages to using VSAM alternate indexes. Performance degradation can occur due to the overhead involved in maintaining and updating the indexes. The update logic for complex data structures may also become more complicated when using alternate indexes.

To create a VSAM alternate index, the IDCAMS utility is used. The DEFINE ALTERNATEINDEX command is employed to define the alternate index, specify its attributes, and allocate space for it. Parameters such as the name of the alternate index, the base cluster it relates to, the key structure, record size, storage location, and other options need to be specified in the command.

For more detailed information on creating a VSAM alternate index, you can refer to the provided example.

VSAM Alternate Index: Merits and Demerits

Advantages

• Can be defined for both KSDS and ESDS
• reduce data redundancy
• Can have duplicates
• Easy to define using IDCAMS
• allow datasets to be accessed by both sequentially and randomly

Disadvantages

• Performance degradation
• Complex update logic

How to create VSAM Alternate Index

//DEFAIX1  JOB   TESTJOB
//STEP1    EXEC  PGM=IDCAMS
//SYSPRINT DD    SYSOUT=A
//SYSIN    DD    *
     DEFINE ALTERNATEINDEX -
           (NAME(EXAMPLE.AIX) -
           RELATE(EXAMPLE.KSDS2) -
           KEYS(3 0) -
           RECORDSIZE(40 50) -
           VOLUMES(VSER01) -
           CYLINDERS(3 1) -
           NONUNIQUEKEY -
           UPGRADE) -
           CATALOG(USERCAT)
/*

The DEFINE ALTERNATEINDEX command creates an alternate index entry, a data entry, and an index entry to define the alternate index EXAMPLE.AIX. The DEFINE ALTERNATEINDEX command also obtains space for the alternate index from one of the VSAM data spaces on volume VSER01, and allocates three cylinders for the alternate index’s use. The parameters are:

• NAME indicates that the alternate index’s name is EXAMPLE.AIX.
• RELATE identifies the alternate index base cluster, EXAMPLE.KSDS2.
• KEYS identifies the length and location of the alternate key field in each of the base cluster’s data records. The alternate key field is the first three bytes of each data record.
• RECORDSIZE specifies that the alternate index’s records are variable length, with an average size of 40 bytes and a maximum size of 50 bytes.
• VOLUMES indicates that the alternate index is to reside on volume VSER01. This example assumes that the volume is already cataloged in the user catalog, USERCAT.
• CYLINDERS allocates three cylinders for the alternate index’s space. The alternate index is extended in increments of one cylinder.
• NONUNIQUEKEY specifies that the alternate key value might be the same for two or more data records in the base cluster.
• UPGRADE specifies that the alternate index is opened by VSAM and upgraded each time the base cluster is opened for processing.
• CATALOG defines the alternate index in the user catalog, USERCAT.

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