Changes an existing global field definition. When you use the
    CHANGE FIELD statement to change the attributes of a global
    field, Oracle Rdb changes only the attributes you mention in the
    statement. All other attributes remain the same.

    Example:

    RDO>  CHANGE FIELD POSTAL_CODE
    cont>    DATATYPE IS TEXT SIZE IS 9 CHARACTERS.

    The CHANGE FIELD statement changes the field definition:

    o  In all the relations that refer to that field

    o  In the data dictionary (if you use INVOKE with the PATHNAME
       qualifier)

    o  In the Oracle Rdb system relations