mysql索引问题

$query = $this->db->query('SELECT * , comments.rank1+comments.rank2+comments.rank3+comments.rank4 as ranksum,sites.id as id FROM sites inner join comments on sites.siteurl=comments.siteurl  WHERE  sites.tj<3 and sites.isshow=1 order by ranksum desc limit 10');
return $query->result_array();

comments.rank是针对站点的评价，每个页面都有这一部分：在页面右侧显示评价总分最高的10个站点。
但是每个页面都做这么一次查询是不是不太好啊，能不能写成索引的形式，求助大神

---

SELECT
    *,
    comments.rank1 + comments.rank2 + comments.rank3 + comments.rank4 AS ranksum,
    sites.id AS id
FROM
    sites
INNER JOIN comments ON sites.siteurl = comments.siteurl
WHERE
    sites.tj < 3
AND sites.isshow = 1
ORDER BY
    ranksum DESC
LIMIT 10