Back to the original topic (BTW -- I like the little guy you put there;-)), here is my answers to your questions:

2) No, I don't have any bitmap index created on that table, only B*tree index. That's why I am kind of confused when I saw all the conversions from bitmap to row_id. Does it look normal to you?

2.1) Actually, CallDirection is an ideal column to create Bitmap index on it since it only has 3 distince values -- inbound, outbound and unknown. So shall I create a bitmap index on it? If I decided to partition this table, do I need to rebuild the bitmap index whenever I drop a partition from the table?

2.2) Since Oracle does combine B*tree indexes on seperate columns, how can we judge the selectivity of composite index and separate column index?

In your previous post, you said the one offers better selectivity usually works better.

3) In my situation, I can't put hint on my queries since they are generated by Crystal report during runtime. So what can I do if CBO didn't use index even if the number of rows returned is <<5% of the total rows in that partition?
FYI -- they are local partitioned index, and have been analyzed.

How can I change DB_File_Multiple_read_count to make CBO change its mind?

Thanks again...