$this->session->sess_destroy(); didn't destroy all session?

[eluser]Alpha[/eluser]
hi, i have $this->session->set_userdata('user_id', 'ID1');

then i want to logout and used $this->session->sess_destroy();
but to my surprise $this->session->set_userdata('user_id', 'ID1') still exist, see below how i used $this->session->sess_destroy();

$this->load->view('index', $data);
$this->session->sess_destroy();

tnx