Partitioning issues

What happen to CBO when you exchanged partition with table ?
If you have a system where you are adding new partition daily, weekly or monthly, do you gather statistics on new partition only or on whole table ? A lot of people do it on partition level only, and this is enough if you select statement is accessing only one partition.
But what happen if you select statement is accessing more then one partition ? Which statistics are used by CBO to generate execution plan ?
Please visit my blog soon and I provide you some examples.