Cause: The indexed function uses SYSDATE or the user environment. See the solution below.
Error solution: PL/SQL functions must be pure (RNDS, RNPS, WNDS, WNPS). SQL expressions must not use SYSDATE, USER, USERENV(), or anything else dependent on the session state. NLS-dependent functions are OK.
Database version: Oracle 11g Release 2 (11.2)