1. A machine-implemented method, comprising: receiving, at a database server, a database query that has a WHERE clause containing a reference to a column, wherein the WHERE clause does not contain any reference to a function on which a functional index on the column is based; wherein the reference to the column occurs in a first condition in the WHERE clause of the database query; the database server creating an amended database query that includes at least one WHERE clause, wherein the at least one WHERE clause includes at least: (a) the first condition, and (b) an added second condition that contains a function call to the function; executing the amended database query; based on the function call to the function in the at least one WHERE clause of the amended database query, using the functional index as an index during execution of the amended database query; and outputting a result set based, at least in part, on the execution of the amended database query; wherein the column is a column of a table in a database that is managed by the database server; wherein the method is performed by one or more computing devices.