Joined: 05 May 2005 Posts: 98 Location: Navi Mumbai, India
Hope the following note helps you -
An Alternate Index AIX provides a view of data different from the one offered by the primary key. For example for a KSDS dataset Employee, you may have a Record Key index on Employee-no and an Alternate Index on Employee-Name . You can now browse and even update the same KSDS in logical sequence by Employee-Name.
Alternate Indexes may be defined on one or more than one Alternate Key(s) i.e. Field(s) other than primary key. Alternate Key(s) need not be unique. Each alternate index itself is a KSDS with data and index component.
Alternate Index greatly reduces redundancy. There is no need to keep a separate dataset for different views like Employees? Social Security No. The records may be accessed sequentially or randomly based on the alternate record keys.
They can be updated automatically when the base cluster is updated.
Alternate Indexes do not support a reusable base cluster. So NOREUSE which is the default, should be specified.
Too many Alternate Indexes built on a KSDS may lead to performance Degradation as access by alternate key requires twice as many I/O?s . VSAM first locates the primary key from the alternate index and then locates the Control Interval information from the record key index.