Index On Partitioning Key

There is a very large fact table that is range partitioned by a column DATE_KEY of type NUMBER(38), such that every hour is a different partition. There is a bitmap index BX$FACT#DATE_KEY on field DATE_KEY, which also is a foreign key referencing DATE_DIM (DATE_KEY). There is a different DATE_KEY for every hour, generated as YYYYMMDDHH24.

When I run

"SELECT * FROM FACT WHERE DATE_KEY >= 2012031207 AND DATE_KEY < 2012031208"

to get all the records for 7 am on March 12th, partition pruning kicks in and sees that only one partition is used. The CBO then decides to do a full scan of the partition. This behavior is correct/desired.

If, however, I run

"SELECT * FROM FACT WHERE DATE_KEY = 2012031207 AND DATE_KEY < 2012031209"

to get all the records for 7 and 8 am, Oracle knows that it will have to scan two partitions. The CBO then decides that using the BX$FACT#DATE_KEY must be a good idea and, instead of doing a full scan of the partitions, does access by local index rowid, which is many times slower.

I think I understand the cause - when more than a single partition is involved, Oracle has to use the global index stats (instead of the local ones, like in the first scenario) and the CBO decides to use it because that the selectivity for the global index is great, when in fact the query will return all the rows for that particular partition (no selectivity).

How to get the CBO to choose a full scan in the second scenario as well? I need to support ad-hoc queries generated by a BI tool, so I cannot add hints to the queries. I also can't get rid of the index on DATE_KEY, because in real life the predicates are on fields of the date dimension, not directly on the key, so I need to join on it.