A compression clause for text or varying text index keys that
    limits the number of characters used for retrieving data. The n
    specifies the number of characters of the key that are used in
    the index.

                                   NOTE

       Although you can create a SIZE IS index and specify the
       UNIQUE clause, truncating the index key values may make the
       key values non-unique. In this case, the index definition or
       insert or update statements fail.