I'll be giving two one hour presentations at the upcoming UKOUG conference 2009 at Birmingham (30th November - 2nd December):
1. Monday, 30th November, 16:00-17:00: "CBO fundamentals: Understanding the different modes of System Statistics"
2. Wednesday, 2nd December, 11:05-12:05: "Everything you always wanted to know about FIRST_ROWS_N but were afraid to ask"
Both presentations cover fundamental functionality of the Cost-Based Optimizer which will help you to better understand why and how the optimizer comes up with certain execution plans, and in particular how the underlying cost calculation works. So eventually, if you haven't got an explanation yet I'll show you what the "cost" calculated effectively means and how to appropriately use the different optimization modes (ALL_ROWS, FIRST_ROWS, FIRST_ROWS_n) available.
So if you happen to attend the conference I'm looking forward to meeting you there.
By the way, I recommend visiting this link and save your personalised agenda to help with the room planning.