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Substitute Key Access Structure Disclosure Number: IPCOM000193140D
Original Publication Date: 2010-Feb-11
Included in the Prior Art Database: 2010-Feb-11
Document File: 2 page(s) / 66K

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Today, VSAM uses the index to retrieve a record from the data component using the key value for the request record. The key determines the position of the record in the data set. VSAM divides the index into CI levels to speed up the search of a key. Each level contains pointers to the next lower level. The sequence set is the lowest level and contains a pointer to the data CI. Every logical record in the sequence set contains high key information for each data CI. The higher level of the index is called the index set. Each CI in the index set contains pointer and high key information for the next lower level of the index.

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Substitute Key Access Structure

If the high key pointer becomes corrupted, we can search sequentially through the index for a key value, but this can take a significant amount of time and I/O. Also, once an index is corrupted, the only way to rebuild that index is to completely delete the data set, redefine it, and put all the record sequentially back into the data set. A better solution would be to have a second index set, which has a pointer to the lowest key in the next lower level of the index. This second index set can be used when the primary index becomes broken. It can also be used to repair the primary index.

The substitute index will keep track of the lowest key value in the next lower level of the index. This is the opposite of what the current primary index does, which keeps track of the highest key in the next lower index value. This substitute index will be a completely separate index set, that will be stored in a different component of the keyed VSAM file. Today, a keyed VSAM file is made up of an index and a data component. With this implementation, the keyed VSAM file will have the same primary index which contains an index set and sequence set, a secondary index which just contains an index set that points to the sequence set of the primary index, and the same data component. So, the difference is the secondary index component.

Once in place, when an error such as a record not found occurs, a secondary search would take place using the sub...