Change Database connection at runtime in Codeigniter

I want to connect to a different database based on the sub domain, Currently I've this code, but it not looking a decent solution to the problem. Please guide in right direction what is the best way to achieve this in code igniter.

class DBConnection extends CI_Model
{
    public function __construct()
    {
        parent::__construct();
    }

    public function dbConfig(){

        $efg="";
        $data = explode('.',$_SERVER['SERVER_NAME']);
        if (!empty($data[0])) {
            $efg = $data[0];
        }

        $sql="SELECT DbUsername,DbName,DbPassword FROM abc WHERE efg=?";
        $d_result=$this->db->query($sql,array($efg))->result_array();

        $this->db->close();

        $config['hostname'] = "localhost";
        $config['username'] = $d_result[0]["DbUsername"];
        $config['password'] = $d_result[0]["DbPassword"];
        $config['database'] = $d_result[0]["DbName"];
        $config['dbdriver'] = "mysql";
        $config['dbprefix'] = "";
        $config['pconnect'] = FALSE;
        $config['db_debug'] = TRUE;
        $config['cache_on'] = FALSE;
        $config['cachedir'] = "";
        $config['char_set'] = "utf8";
        $config['dbcollat'] = "utf8_general_ci";
        return $config;
    }
} 

Then in User model class, I've something like this, which is on each & every request doing all the above processing again. I want the switch will take place in the start and further queries will push towards selected database.

class User extends CI_Model
{
    private $clientDB;

    public function __construct()
    {
        parent::__construct();
        $db=new DBConnection();
        $this->clientDB=$this->load->database($db->dbConfig(),TRUE);
    }

    public function isUserExists($username,$password)
    {
        $sql="SELECT uid FROM `aaa` WHERE `uname`=? AND `upwd`=?";
        $d_result=$this->clientDB->query($sql,array($username,$password));
        return $d_result;
    }
}