Bitmap index costing - how to influence

I am pretty much stuck on a bitmap costing problem.

I have a query on a table with 10M rows (709MB), and the predicates are correctly estimated to return 800 000 rows. As a result with the famous 80/20 split, my bitmap index access path cost is ~170 000. The full table scan cost is ~20 000. Obviously Oracle choose FTS.

When executed with a hint, the query touches about 8000 block from the table, and needless to say, is significantly faster.

Any ideas on how to influence bitmap index cost, relative to full table scan cost? Anything goes as long as it doesn't break b-tree index costing.

Thing's I've already considered, but somewhat ruled out
- adjust "mbrc" system statistic - event at max (128) cost is still
133 000 vs 16 000 fts
- optimizer_index_cost_adj - i will have to set this to 10 (10 times
cheaper) for bitmaps to be used. I am concerned about proper calculations of b-tree indexes vs full table scans

Any other ideas?

-- 
Christo Kutrovsky
Senior DBA
The Pythian Group - www.pythian.com
I blog at http://www.pythian.com/blogs/
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http://www.freelists.org/webpage/oracle-l