错误508.达到资源限制

Question

public function get_advertisement($userid){
    $query="SELECT *
            FROM
            advertisements
            WHERE
            advertisements.adtype =  'website'
            HAVING
            advertisements.id NOT IN  (SELECT
            work_done_ads.ad_id
            FROM
            work_done_ads
            WHERE
            work_done_ads.user_id =  '".$userid."')
            order by advertisements.id asc
            LIMIT 1";
        $result= $this->db->query($query)->row();
        $query = $this->db->query("SELECT *
            FROM
            advertisements
            WHERE
            advertisements.adtype =  'website'
            HAVING
            advertisements.id NOT IN  (SELECT
            work_done_ads.ad_id
            FROM
            work_done_ads
            WHERE
            work_done_ads.user_id =  '".$userid."')
            order by advertisements.id asc
            LIMIT 1");
        if ($query->num_rows() > 0) {
        foreach ($query->result_array() as $row)
            {
                return $result;
            }
        } else {
            $query = $this->db->query("SELECT ad_cycle from users WHERE userid='$userid' LIMIT 1");
            if ($query->num_rows() > 0) {
            foreach ($query->result_array() as $row)
                {
                    $ad_cycle = $row["ad_cycle"];
                    $ad_cycle = $ad_cycle + 1;
                }
            }


            $this->db->query("UPDATE users SET ad_cycle=$ad_cycle WHERE userid='$userid' LIMIT 1");
            $this->db->query("DELETE FROM work_done_ads WHERE user_id='$userid'");
            $query="SELECT *
                FROM
                advertisements
                WHERE
                advertisements.adtype =  'website'
                HAVING
                advertisements.id NOT IN  (SELECT
                work_done_ads.ad_id
                FROM
                work_done_ads
                WHERE
                work_done_ads.user_id =  '".$userid."')
                order by advertisements.id asc
                LIMIT 1";
                $result= $this->db->query($query)->row();
                return $result;
        }

}

如果用户完成所有广告而不是视图计数器设置为0，则此查询会从用户尚未看到的数据库中获取广告，以便用户可以继续看到广告，这是一个持续的循环。

此查询使用太多资源，服务器提供508错误，有没有办法编写使用较少服务器资源的此查询