no segment, how redirect 404 ?

Be sure to properly validate your input! Although you process a _GET variable which should be validated by the permitted_ui_chars in your config but if by some fuck-up gets changed to allow all chars you are in trouble.

In this case I would simply type cast the $value to an integer. Or use query bindings to escape the value in your query like in the example below.

Also if you are just interested in knowing if a record with some field value exists, dont't use "select *", but "select 1".

PHP Code:

public function check_value($value)
    {
        $sql    = "SELECT 1 FROM table_name WHERE field = ? ";
        $query  = $this->db->query($sql, array($value) );

        $result = $query->result_array();

        if (empty($result)) {
            return false;
        }

        return true;

    }