Every version of Oracle optimizer has new features and bugs. As requirements of every Oracle environment varies, there may be a need to enable and disable these features or bugs. From Oracle version 10.2.0.2 , Oracle introduced underscore parameter “_FIX_CONTROL” to enable and disable certain features or bugs mainly related to optimizer. This parameter can be specified at session or system level or at init.ora level.
The syntax is “_FIX_CONTROL”=’BUG#:OFF’ or “_FIX_CONTROL”=’BUG#:ON’. Incorrect syntax may prevent the database from being started or cause hangs if issued with alter system command.
Any patch that you want to disable or enable must be installed and viewable in the V$SYSTEM_FIX_CONTROL. The view V$SYSTEM_FIX_CONTROL provides brief description of the patch and the Oracle version it was introduced. At session level , you can query V$SESSION_FIX_CONTROL.
Note: Please consult with Oracle support before setting this parameter