codeigniter how to know update failed

1$this->db->trans_start();
2$this->db->query('AN SQL QUERY...');
3$this->db->update('table',$array);
4$this->db->trans_complete();
5
6if ($this->db->trans_status() === FALSE)
7{
8    // generate an error... or use the log_message() function to log your error
9}