Materialized views - query rewrite -- optional joins & dimensions

Question

Hi,
I've implemented a number of materialized views that are accessible via query rewrite.
Most of these views make use of duplicate tables (DD_TIME table is joined via purchase, load, process, ... date columns)
I've also created a dimension to make Oracle aware of the time relation (month is child of year...)
The issue:
--> If I create the mat. view with inner joins, query rewrite works with the dimensions, but I need to provide ALL joined tables in the query
--> If I create the mat. view with outer joins, query rewrite works even with 1 joined table, but the dimension seems to be ignored
How can I create 'optional' joins in the Mat. View while making use of dimensions for rollup?
(the limitations on materialized view delta joins mentioned on [oracle docs|http://docs.oracle.com/cd/B28359_01/server.111/b28313/qradv.htm#autoId16] kinda has me worried)
Example Mat. View:
CREATE MATERIALIZED VIEW DA_REPOSITORY_MERCHANT_MM
... ENABLE QUERY REWRITE
AS SELECT
DDFPC.YEAR_NO,
DDFPC.YEAR_MONTH,
DDPUR.YEAR_NO,
DDPUR.YEAR_MONTH,
SUM( DF.TRANSACTION_AMOUNT )
FROM DF_REP DF,
DD_TIME DDFPC,
DD_TIME DDPUR
WHERE DF.FPC_DTE = DDFPC.DAY_DATE(+)
AND DF.PUR_DTE = DDPUR.DAY_DATE(+)
GROUP BY
DDFPC.YEAR_NO,
DDFPC.YEAR_MONTH,
DDPUR.YEAR_NO,
DDPUR.YEAR_MONTH
Example query launched:
SELECT
DDFPC.YEAR_NO,
SUM( DF.TRANSACTION_AMOUNT )
FROM DF_REP DF,
DD_TIME DDFPC
WHERE DF.FPC_DTE = DDFPC.DAY_DATE
GROUP BY
DDFPC.YEAR_NO

Answer 1

The problem was resolved by setting foreign keys in the fact table and making them not nullable + using inner joins.
--> this informs oracle there can be no difference between a query without the dimensions and a query with the dimensions.
As a result, the materialized view can contain 1 fact linked to 2 dimension tables, but the query rewrite will work for queries on the 1 fact joined with only 1 dimension