There are three distinct AMS commands to create an alternate index:
To ensure that the access of records through an alternate index is accurate, the alternate index has to be kept in sync with the base cluster, i.e. whenever a record is added, deleted or modified in the base cluster, the appropriate changes have to be made for the alternate index.
Now, the responsibility of maintaining the alternate index can be done manually be deleting the alternate index, allocating them and reloading them with the BLDINDEX command. The drawback of this approach is that there will still be times when the alternate index and the base cluster are not in sync. Another alternative is to have the alternate index automatically updated to reflect the changes in the base cluster. This can be accomplished by specifying an UPGRADE parameter while creating the alternate index. Alternate indexes thus created are said to belong to the upgrade set of the base cluster.
The disadvantage of the latter option is the extra overhead involved each time an update is made to the base cluster. Such overheads could prove to be crucial in a response-critical online application. However, there could be applications that need the alternate index to be always in sync with the base cluster. The trade¬off has to be made depending on the nature of the individual applications.
DEFINE AIX Command
As mentioned before DEFINE AIX allocates space for an alternate index cluster. Before defining an alternate index it should be noted that the base cluster has to be defined first, though not necessarily loaded.
We will use our CUSTOMER.KSDS.CLUSTER for which the first 6 positions indicate the customer number, which is the prime key, and the next 10 positions indicate the customer name on which we will define the alternate index.
If this JCL were executed after being customized to your installation standards, an AIX cluster CUSTOMER.KSDS.AIX1.CLUSTER would be allocated. The allocation messages would be in the SYSPRINT device. Note that we have just created the AIX cluster and it will still be empty. You would have to load records into it by using the BLDINDEX command. This will be discussed in the next section. Note that there are more extensive options for allocation of an AIX cluster. Refer to an IBM manual for details.
This command loads records into an AIX cluster. Prior to executing this command, the base cluster must be defined, loaded with records and the AIX cluster must be defined. When the BLDINDEX command is executed, three distinct functions occur in the following order.
Records of the base cluster are read and alternate key - pointer pair records are extracted
These key - pointer pair records are sorted in ascending order of the alternate key.
The alternate index is loaded with these sorted pair records.
The sample JCL given above loads the CUSTOMER.KSDS.AIX1 .CLUSTER.
This command establishes a relation between the base cluster and the alternate index. A path is not a data set. It does not occupy any space, nor does it have any records. It is just an entry in the catalog, which establishes a link between the base cluster and the alternate index cluster. Below is a sample JCL for creating a PATH
PATHENTRY specifies the alternate index cluster to which the path is related.
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The Dd Statement
Procedures And Symbolic Parameters
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Access Method Services (ams)
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Introduction To Database Management Systems
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Overview Of Information Management System (ims)
Introduction To Vs Cobol Ii
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