If "documented" does not strictly mean "in official documentation", Note:377333.1, last modified in 2006, serves as proof it's documented. The note does not have warning about any harmful side effect. Also pay attention to the bottom of the note: "Bug 5160959 - THE HINT OPT_PARAM SHOULD BE DOCUMENTED" Yong Huang > The opt_param hint is documented, albeit only in the Oracle 11i SQL > Reference Manual even though it is also available in 10g (R2 at > least, haven't tried it in 10.1.x ): -- //www.freelists.org/webpage/oracle-l