Simple answer is that a postgresql 'function' is not the same as a jdbc stored procedure. Just try to write a jdbc CallableStatement and execute it using jdbc. And what happens? That is all this jpa StoredProcedureQuery does.

If it works using jdbc then you can easily get the code for PostgreSQLAdapter in datanucleus-rdbms and develop itband contribute it back.

If it doesnt work with jdbc then execute a native query to execute the function as you would normally with postgresql