I found this too

http://download-west.oracle.com/docs...0/mv.htm#36835


Materialized View Restrictions
You should keep in mind the following restrictions:

The defining query of the materialized view cannot contain any non-repeatable expressions (ROWNUM, SYSDATE, non-repeatable PL/SQL functions, and so on).

The query cannot contain any references to RAW or LONG RAW datatypes or object REFs.

If the defining query of the materialized view contains set operators (UNION, MINUS, and so on), rewrite will use them for full text match rewrite only.

If the materialized view was registered as PREBUILT, the precision of the columns must agree with the precision of the corresponding SELECT expressions unless overridden by the WITH REDUCED PRECISION clause.

If the materialized view contains the same table more than once, it is possible to do a general rewrite, provided the query has the same aliases for the duplicate tables as the materialized view.


MH