getting message: oci_execute(): ORA-00942: table or view does not exist

from my attached Dep_models.php, here I am using 'oracle' and the 'default' line is remarked:

PHP Code:

   public function __construct() //model construct function
    {
        $this->db=$this->load->database('oracle',true); // oracle db
        //$this->db=$this->load->database('default',true); // mysql db
    }

    public function getDept()
    {
        $query=$this->db->get('departments');
        //$query=$this->db->get('departments');
        return $query;
    }