public void clear() {
    lock.lock();
    try {
      if (isNative) {
        throw new IllegalStateException("System function registry cannot be cleared");
      }
      mFunctions.clear();
      builtIns.clear();
      persistent.clear();
    } finally {
      lock.unlock();
    }
  }
Unregisters all the functions belonging to the specified database
@param dbName database name
@throws HiveException
ql/src/java/org/apache/hadoop/hive/ql/exec/Registry.java