5.3.6 Setting a null-value exclusion specification (EXCLUDE NULL VALUES)
If you set a null-value exclusion specification when you define a B-tree index, a B-tree index is created that does not contain keys comprised of null values only (keys comprised of null values only are not created).
For example, if you set as an indexed column one that is comprised mostly of null values, consider setting a null-value exclusion specification to the index. When you set the null-value exclusion specification, keys comprised of null values only are not created, and therefore you can shorten the time required for creating a B-tree index. You can also reduce the data size of the B-tree index.
You can set the null-value exclusion specification by specifying EXCLUDE NULL VALUES when you use the CREATE INDEX statement to define a B-tree index.
Note that if the table retrieval range includes null values, a B-tree index for which the null-value exclusion specification is specified does not become a candidate index to be used for the retrieval. Examples include cases in which no search condition is specified, and cases in which the IS NULL predicate is specified.