Materialzed view rewrite

Question

Hi
http://download.oracle.com/docs/cd/B28359_01/server.111/b28313/qradv.htm#CHDFIAGB
In the following section
Materialized View Delta Joins
A materialized view delta join is a join that appears in the materialized view but not the query. All delta joins in a materialized view are required to be lossless with respect to the result of common joins. A lossless join guarantees that the result of common joins is not restricted. A lossless join is one where, if two tables called A and B are joined together, rows in table A will always match with rows in table B and no data will be lost, hence the term lossless join. For example, every row with the foreign key matches a row with a primary key provided no nulls are allowed in the foreign key. Therefore, to guarantee a lossless join, it is necessary to have FOREIGN KEY, PRIMARY KEY, and NOT NULL constraints on appropriate join keys. Alternatively, if the join between tables A and B is an outer join (A being the outer table), it is lossless as it preserves all rows of table A.
why we need lossless joins?
why we need non duplicating joins?
does we need both to perform query rewrite or anyone of them
thanks
Nick

Answer 1

Assume you have a query that ends up looking like:
select * from A where ...and there is a materialized view looking like:
select A.*, B.* from A, B where ...If the join from A to B causes rows from A to appear multiple times (imagine A is "orders" and B is "order_lines" then the materialized will give you the wrong result. This answers the bit about duplicates.
If the join from A to B causes rows from A to disappear (imagine A is the order_lines and B is the orders this time, but there is a predicate on B in the materialized view that restricts the views to 'completed' orders) then you get the wrong result. This answers the bit about lossless joins.
You might also want to read the following blog item from Oracle's optimizer group about "join elimination" as the sample principle applies.
http://optimizermagic.blogspot.com/2008/06/why-are-some-of-tables-in-my-query.html
Regards
Jonathan Lewis
http://jonathanlewis.wordpress.com
http://www.jlcomp.demon.co.uk
"The greatest enemy of knowledge is not ignorance,
it is the illusion of knowledge." (Stephen Hawking)