thanks Hirshy.

here is the latest test I have.

I trace two different execution plans one with Optimizer_index_cost_adj =10 and one without Optimizer_index_cost_adj =10
and below is what I find:

on the total CPU time on the trace file on Optimizer_index_cost_adj =10 is much lowever than the one without Optimizer_index_cost_adj =10. but I what I don;t understand is the top 5 events of CPU is higher than before:

Code:
Top 5 Timed Events
~~~~~~~~~~~~~~~~~~                                                     % Total
Event                                               Waits    Time (s) Ela Time
-------------------------------------------- ------------ ----------- --------
CPU time                                                           21    92.13
db file sequential read                            28,842           1     5.83
control file parallel write                           497           0     1.81
SQL*Net more data to client                           325           0      .09
log file sync                                           1           0      .06
could you please explain to me why???