"alter session set sql_trace" Privilege

I can do this:
klabu_at_devzone> alter session set nls_date_format='dd-mon-yyyy hh24:mi:ss' ;

Session altered.

But NOT:

klabu_at_devzone> alter session set sql_trace=true ; alter session set sql_trace=true
*
ERROR at line 1:
ORA-01031: insufficient privileges

Why ? It seems I can "alter session" but why "alter session set sql_trace=true" failed ?
And How can I enable sql_trace ?
Thx Received on Mon Oct 02 2006 - 10:22:38 CDT