SET FIRSTVAL or SET NOFIRSTVAL overrides the setting on the opf_like_firstval configuration parameter in a session.

The opf_like_firstval parameter controls how the DBMS optimizes REPEATED queries with a LIKE expression involving a program variable. If ON the value of the variable when the QEP is (re-)generated is used to determine the selectivity of that expression. If OFF no value is used and default selectivity rules are used. This should be set to ON only if the first value is representative of the subsequent values that may be encountered for the same query. 

The default is OFF.