Oracle Message QSM-02058 - set operator in mv and compatibility mode is less than 9.0

ERRORset operator in mv and compatibility mode is less than 9.0
CAUSEThe capability in question is not supported when the materialized view uses a set operator and the compatibility mode is less than 9.0.
ACTIONRe-phrase the query to avoid the use of set operators or set the compatibility mode to 9.0 or greater.