Basic session Problem

[eluser]ajay009ajay[/eluser]
Yes i read it properly and here is my implementation

if($query->num_rows() > 0 ){

$row = $query->row();
$id=$row->id;

$this->session->set_userdata($id); // Need to understand here //


$session_id = $this->session->userdata('session_id'); // Need to understand here

echo $session_id;
echo "aaaaaaaaaaaaaaaaaaaaaaaa";

}

I want to enable session when if statement will be true.
Am i doing ok?