Creates an index for a relation. An index allows Oracle Rdb direct
    access to the records in the relation, to avoid sequential
    searching. You can define a sorted index or a hashed index. A
    sorted index uses the B-tree method of retrieval. A hashed index
    uses hash addressing for exact match retrievals.

    Example

      RDO>  DEFINE INDEX EMP_EMPLOYEE_ID FOR EMPLOYEES
      cont>   DUPLICATES ARE NOT ALLOWED
      cont>   TYPE IS SORTED.
      cont>     EMPLOYEE_ID.
      cont>  END EMP_EMPLOYEE_ID INDEX.