About query rewriting

Question

Hi,
I just started using materialized view and found in trouble.
In my case the query rewriting works only if the tables joined in the query are exactly the same
of those joined in the MV.
Is this normal?
Using EXPLAIN_MVIEW I found that when rewriting doesn't happen I got:
QSM-01284: materialized view <string> has an anchor table <string> not found in query
IE:
MV is select <some fields> from t1, t2, t3 where t1.id = t2.fk and t1.id = t3.fk group by <some fields>
query1 is select <few fields> from t1, t2, t3 where t1.id = t2.fk and t1.id = t3.fk group by <few fields>
query2 is select <few fields> from t1, t2 where t1.id = t2.fk group by <few fields>
The rewrite works query1 but not for query2.
Could you explain me why?
Thanks

Answer 1

The first condition to use any query rewrite is to be sure that MV contains data that can be (at least partially) used for your query X you'd like to rewrite. For your 2nd query MV doesn't contain any usable data because joining another table generally further restricts result set and one cannot use these data to get result from less joined result set. It is simply not possible. Of course group by clause goves another complication in your scenario, but the first condition anyway remains.
Gints Plivna
http://www.gplivna.eu