VSAM key-sequenced data sources support the use of alternate key indexes (keys). A key-sequenced VSAM data source consists of two components: an index component and a data component. The data component contains the actual data records, while the index component is the key used to locate the data records in the data source. Together, these components are referred to as the base cluster.
An alternate index is a separate, additional index structure that enables you to access records in a KSDS VSAM data source based on a key other than the data source primary key. For instance, you may usually use a personnel data source sequenced by Social Security number, but occasionally need to retrieve records sorted by job description. The job description field might be described as an alternate index. An alternate index must be related to the base cluster it describes by a path, which is stored in a separate data source.
The alternate index is a VSAM structure and is, consequently, created and maintained in the VSAM environment. It can, however, be described in your Master File, so that you can take advantage of the benefits of an alternate index.
The primary benefit of these indexes is improved efficiency. You can use it as an alternate, more efficient, retrieval sequence or take advantage of its potential indirectly, with screening tests (IF...LT, IF...LE, IF...GT, IF...GE, IF...EQ, IF...FROM...TO, IF...IS), which are translated into direct reads against the alternate index. You can also join data sources with the JOIN command through this alternate index.
It is not necessary to identify the indexed view explicitly in order to take advantage of the alternate index. An alternate index is automatically used when described in the Master File.
To take advantage of a specific alternate index during a TABLE request, provide a WHERE or IF test on the alternative index field that meets the above criteria. For example:
TABLE FILE CUST PRINT SSN WHERE LNAME EQ 'SMITH' END
As you see in the Master File in Describing a VSAM Alternate Index, the LNAME field is defined as an alternate index field. The records in the data source are retrieved according to their last names, and certain IF screens on the field LNAME result in direct reads. Note that if the alternate index field name is omitted, the primary key (if there is any) is used for a sequential or a direct read, and the alternate indexes are treated as regular fields.
Alternate indexes must be described in the Master File as fields with FIELDTYPE=I. The ALIAS of the alternate index field must be the file name allocated to the corresponding path name. Alternate indexes can be described as GROUPs if they consist of portions with dissimilar formats. Remember that ALIAS=KEY must be used to describe the primary key.
Only one record type can be referred to in the request when using alternate indexes, but there is no restriction on the number of OCCURS segments.
Note that the path name in the allocation is different from both the cluster name and the alternate index name.
If you are not sure of the path names and alternate indexes associated with a given base cluster, you can use the IDCAMS utility. (See the IBM manual entitled Using VSAM Commands and Macros for details.)
Consider the following:
FILENAME = CUST, SUFFIX = VSAM,$ SEGNAME = ROOT, SEGTYPE = S0,$ GROUP = G, ALIAS = KEY, A10, A10,$ FIELD = SSN, SSN, A10, A10,$ FIELD = FNAME, DD1, A10, A10, FIELDTYPE=I,$ FIELD = LNAME, DD2, A10, A10, FIELDTYPE=I,$
In this example, SSN is a primary key and FNAME and LNAME are alternate indexes. The path data set must be allocated to the ddname specified in ALIAS= of your alternate index field. In this Master File, ALIAS=DD1 and ALIAS=DD2 each have an allocation pointing to the path data set. FNAME and LNAME must have INDEX=I or FIELDTYPE=I coded in the Master File. CUST must be allocated to the base cluster.
The following example demonstrates how to use IDCAMS to find the alternate index and path names associated with a base cluster named CUST.DATA:
First, find the alternate index names (AIX) associated with the given cluster.
IDCAMS input: LISTCAT CLUSTER ENTRIES(CUST.DATA) ALL IDCAMS output (fragments): CLUSTER -------- CUST.DATA ASSOCIATIONS AIX ---------- CUST.INDEX1 AIX ---------- CUST.INDEX2
This gives you the names of the alternate indexes (AIX): CUST.INDEX1 and CUST.INDEX2.
Next, find the path names associated with the given AIX name:
IDCAMS input: LISTCAT AIX ENTRIES (CUST.INDEX1 CUST.INDEX2) ALL IDCAMS output (fragments): AIX ---------CUST.INDEX1 ASSOCIATIONS CLUSTER -- CUST.DATA PATH ------CUST.PATH1 AIX ---------CUST.INDEX2 ASSOCIATIONS CLUSTER -- CUST.DATA PATH ------CUST.PATH2
This gives you the path names: CUST.PATH1 and CUST.PATH2.
This information, along with the TSO DDNAME command, may be used to ensure the proper allocation of your alternate index.