Cannot make num_rows work

[eluser]huskyfritz[/eluser]
Hello guys, I am newbie in codeigniter and I am developing a simple site using this framework. Well I am trying to use num_rows to get all records in an array but still no luck.

here's my codes:

Code:

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

Is it correct? Is there any simpliest way to achieve this?

Thanks much!