VSAM Alternate Index: How to Use it

VSAM alternate index merits
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The alternate index in VSAM to access records other than the base key defined on the dataset is called a base cluster.


  • 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


  • Performance degradation
  • Complex update logic

Click here for VSAM alternate index example.

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Author: Srini

Experienced software developer. Skills in Development, Coding, Testing and Debugging. Good Data analytic skills (Data Warehousing and BI). Also skills in Mainframe.