24660 - Using an index for BY processing instead of sorting

Create and use an index for BY-group processing when sorting is not possible or practical.

Note:   If you have a single variable on the BY statement, use a simple index. If you have multiple variables in the BY-group, you can use a simple index on the first variable in the BY-group, or you can use a composite index built on the variables on the BY statement in the same order. A composite index does not have to contain all the variables in the BY-group, but at a minimum the first variable of the BY-group must be the same as the first variable in the index. Additional variables in the composite index should match the order of the variables in the BY-group for best performance. If there are multiple BY variables and the composite index does not include all of them, the unnamed variables must be in sorted order or the BY process will fail.

System performance will not be as good as using a sorted data set, but it will allow you to use a BY statement without having a sorted data set.

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