public function prepare($sql, $driver_options=array())
    {

        $md5 = md5($sql);

        if(isset($this->prepared_sql[$md5])){
            return $this->prepared_sql[$md5];
        }

        $stmt = parent::prepare($sql, $driver_options);
        $this->prepared_sql[$md5] = $stmt;
        return $stmt;
    }