Oracle Error ORA-12013: updatable materialized views must be simple enough to do fast refresh

Oracle Error Message

ORA-12013: updatable materialized views must be simple enough to do fast refresh

Reason for the Error

The updatable materialized view query contained a join, subquery, union, connect by, order by, or group by caluse.

Solution

Make the materialized view simpler. If a join is really needed, make multiple simple materialized views then put a view on top of them.