Get value from DB

[eluser]shahnawazahmed[/eluser]
its my controller
<?php
class site extends CI_controller
{
function index()
{
$this->load->model('getdb');
$a=$this->getdb->getall();
echo $a;


}


}

?>


and here is the model

<?php
class getdb extends CI_Model{

public function getall()
{
echo $query= $this->db->query('SELECT user_role FROM res_user');



}



}

?>

i want to fetch value from the db column user_role ...