Oracle 11g upgrade issues

Question

Hello :
We recently migrated our database from 9.2.07 (RBO) to 11.2.0.1 (CBO) ( Solaris 10 Sparc 64 bit ). Ever since the migration happened we are experiencing the following issues:
1. Execution plans are very costly. ( Please note that we moved from RBO to CBO )
2. High Gets / Reads
3. High CPU Utilization
4. High waits on 'DB FILE SEQUENTIAL READ'
5. This situation is vitually killing all our applications performance.
I know that oracle 11g explicitly does not support RULE based optimizer , but is there any parameters that can force it go the RBO way.
I would really appreciate your help.
Regards,
BMP

Answer 1

Officially it is no more documented http://download.oracle.com/docs/cd/E11882_01/server.112/e10820/initparams165.htm#i1131532
but it is still accepted:
SQL> select * from v$version;
BANNER
Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - Production
PL/SQL Release 11.2.0.1.0 - Production
CORE    11.2.0.1.0      Production
TNS for Linux: Version 11.2.0.1.0 - Production
NLSRTL Version 11.2.0.1.0 - Production
SQL> alter system set optimizer_mode=rule;
System altered.
SQL> show parameter optimizer
NAME                                 TYPE        VALUE
optimizer_capture_sql_plan_baselines boolean     FALSE
optimizer_dynamic_sampling           integer     2
optimizer_features_enable            string      11.2.0.1
optimizer_index_caching              integer     0
optimizer_index_cost_adj             integer     100
optimizer_mode                       string      RULE
optimizer_secure_view_merging        boolean     TRUE
optimizer_use_invisible_indexes      boolean     FALSE
optimizer_use_pending_statistics     boolean     FALSE
optimizer_use_sql_plan_baselines     boolean     TRUEEdited by: P. Forstmann on 1 sept. 2010 20:21