NULL keys can be excluded from indexes

Starting with DB2® Version 10.5, you can use the EXCLUDE NULL KEYS clause on the CREATE INDEX statement to reduce the size of sparse indexes..

An index that you create with the EXCLUDE NULL KEYS clause does not insert a key into the index object when all the columns in the key are null. Excluding null keys can help optimize storage and performance for cases where you do not want queries to access data that is associated with null keys. For unique indexes, the enforcement of the uniqueness of table data causes rows with null index keys to be ignored.