An alternate index provides access to the records in a
data set that uses more than one key. It accesses records in the same
way as the prime index key of an indexed data set (KSDS).
About this task
When planning to use an alternate index, you must know:
- The type of data set (base cluster) with which the index will
be associated
- Whether the keys will be unique or not unique
- Whether the index is to be password protected
- Some of the performance aspects of using alternate indexes
Because
an alternate index is, in practice, a VSAM data set that contains
pointers to the keys of a VSAM data set, you must define the alternate
index and the alternate index path (the entity that establishes the
relationship between the alternate index and the prime index). After
you define an alternate index, make a catalog entry to establish the
relationship (or path) between the alternate index and its base cluster.
This path allows you to access the records of the base cluster through
the alternate keys.
To use an alternate index, do these steps:
Procedure
- Define the alternate index by using the
DEFINE
ALTERNATEINDEX command. In it, specify these items:
- Name of the alternate index
- Name of its related VSAM indexed data set
- Location in the record of any alternate indexes and whether they
are unique
- Whether alternate indexes are to be updated when the data set
is changed
- Name of the catalog to contain this definition and its password
(can use default name)
In your COBOL program, the
alternate index is identified solely by the ALTERNATE RECORD
KEY clause in the FILE-CONTROL paragraph.
The ALTERNATE RECORD KEY definitions must match the
definitions in the catalog entry. Any password entries that you cataloged
should be coded directly after the ALTERNATE RECORD KEY phrase.
- Relate the alternate index to the base cluster (the data
set to which the alternate index gives you access) by using the
DEFINE
PATH command. In it, specify these items:
- Name of the path
- Alternate index to which the path is related
- Name of the catalog that contains the alternate index
The base cluster and alternate index are described by entries
in the same catalog.
- Load the VSAM indexed data set.
- Build the alternate index by using (typically) the
BLDINDEX command.
Identify the input file as the indexed data set (base cluster) and
the output file as the alternate index or its path. BLDINDEX reads
all the records in the VSAM indexed data set (or base cluster) and
extracts the data needed to build the alternate index. Alternatively,
you can use the runtime option AIXBLD to build the
alternate index at run time. However, this option might adversely
affect performance.