difference between $query>num_rows() and $this->db->count_all_results() in CodeIgniter & which one is recommended

Question

In a scenario I need to know the count of recordset a query will return, which in codeigniter can be done by $query->num_rows() or $this->db->cou

Answer 1

$sql = "select count(*) as row from login WHERE firstname = '" . $username . "' AND password = '" . $password . "'";
    $query = $this->db->query($sql);
    print_r($query);exit;
    if ($query->num_rows() == 1) {
    return true;
    } else {
    return false;
    }

Answer 2

We can also use

return $this->db->count_all('table_name');  

or

$this->db->from('table_name');
return $this->db->count_all_result();

or

return $this->db->count_all_result('table_name');

or

$query = $this->db->query('select * from tab');  
return $query->num_rows();