The name of the field(s) that make up the index. You can create a
    multisegment index by naming two or more fields, which are joined
    to form the index. All of the fields must be part of the same
    relation. Separate multiple field names with periods.

                                   NOTE

       If field-name refers to a field defined as VARYING STRING
       data type, the size of the field must be less than or equal
       to 255.