Sparse Keys

A sparse key is a key for which no index entry is stored for a given value of that key. For example, if a key is defined as sparse when it contains all spaces, index entries for the key are not included when the part of the record it occupies contains only space characters.

Only alternate keys can be sparse.

Using this feature results in smaller index files. The larger your key(s) and the more records you have for which the alternate key has the given value, the larger your saving of disk space.

To enable sparse keys, use SUPPRESS WHEN ALL in the ALTERNATE RECORD KEY clause in the SELECT clause:

 file-control.
     select idxfile assign to "idx.dat"
         organization is indexed
         record key is idxfile-record-key
         alternate record key is idxfile-alt-key
                                 with duplicates 
                                 suppress when all "A".

In this example, if a record is written for which the value of the alternate key is all As, the actual key value is not stored in the index file.