How to set string parameter for another user's session?

Hi List,  

I'm trying to figure out a way to alter the optimizer_mode parameter to first_rows for another user's session - is there any way to do this other than a logon trigger? I know about dbms_system.set_int_param_in_session and set_bool_param_in_session, but they only work for integers and booleans, not strings like 'first_rows'.

Thanks,
Brandon  

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