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Module 2: VSAM Concepts


Index component

  • Index component contains the actual index information
  • The index component can be thought of as index in a book, where the chapter name is given along with page number on which it is explained
  • Similarly, Index component of VSAM contains the key field and location of its corresponding records. Thus, Index components makes it easy to retrieve any record from data component
  • Index component consist of two parts:-
    • Sequence set
    • Index set

Sequence set

  • Sequence set is lowermost level of index
  • It contains primary key and pointers directly to the control interval of the data component
  • There is one sequence set for one control area
  • The highest record key of every control interval is stored in sequence set
  • Please refer example given in section “VSAM Splits CI Split and CA Split” that will help you understand Sequence set in more detail

Index set

  • The index set is used for random access
  • The highest record key of the sequence set is stored in first level of index set i.e. first level if index set contains pointers to sequence set. However, highest record key of the first level index set is stored in second level index set i.e. second level of index set contain pointers to first level of index set and this goes on till the uppermost level of index set
  • Based on size of control interval, there will be 1-3 levels of index sets in the index component of the dataset.
  • Please refer example given in section “VSAM Splits CI Split and CA Split” that will help you understand Index set in more detail



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