Cannot make num_rows work

[eluser]huskyfritz[/eluser]
Yes, lots of records.. My code using num_rows is same as it showed in codeigniter documentation.

The documentation says like this:

Code:

$query = $this->db->query('SELECT * FROM my_table');
   echo $query->num_rows();

and here's my code:

Code:

$query_array = $this->db->query('SELECT STATEMENT');
  die($query_array->num_rows());

do u know what's wrong with my code?

Thanks