优化MySQL查询以避免"使用where;使用临时;使用filesort"

Question

我使用MySQL为我的网站建立了一个自定义论坛.列表页面本质上是一个包含以下列的表:主题,上次更新和#Reslies.

DB表包含以下列:

id
name
body
date
topic_id
email

一个主题为"0"的topic_id,并回复有自己的母话题的topic_id.

SELECT SQL_CALC_FOUND_ROWS
    t.id, t.name, MAX(COALESCE(r.date, t.date)) AS date, COUNT(r.id) AS replies
FROM
    wp_pod_tbl_forum t
LEFT OUTER JOIN
    wp_pod_tbl_forum r ON (r.topic_id = t.id)
WHERE
    t.topic_id = 0
GROUP BY
    t.id
ORDER BY
    date DESC LIMIT 0,20;

此表中总共有2,100个项目,查询通常需要6秒钟.我在"topic_id"列中添加了一个INDEX,但这没有多大帮助.有没有办法加速这个查询而不进行重大重组？

编辑:还没有完成工作.我似乎无法让下面的例子正常工作.

Answer 1

SELECT  id, name, last_reply, replies
FROM    (
        SELECT  topic_id, MAX(date) AS last_reply, COUNT(*) AS replies
        FROM    wp_pod_tbl_forum
        GROUP BY
                topic_id
        ) r
JOIN    wp_pod_tbl_forum t
ON      t.topic_id = 0
        AND t.id = r.topic_id
UNION ALL
SELECT  id, name, date, 0
FROM    wp_pod_tbl_forum t
WHERE   NOT EXISTS
        (
        SELECT  NULL
        FROM    wp_pod_tbl_forum r
        WHERE   r.topic_id = t.id
        )
        AND t.topic_id = 0
ORDER BY
       date DESC
LIMIT 0, 20

如果您的表是MyISAM或id不是a PRIMARY KEY,则需要创建复合索引(topic_id, id).

如果你的表是InnoDB和id是PRIMARY KEY,那么只需要一个索引(topic_id)(id将隐式添加到索引中).

更新

这个查询很可能会更加有效,前提是你必须对指标(topic_id, id)和(date, id):

有关性能详情,请参阅我的博客中的这篇文章

• 选择最后的论坛帖子

此查询在行样本数据30 ms上完成100,000:

SELECT  id, name, last_reply,
        (
        SELECT  COUNT(*)
        FROM    wp_pod_tbl_forum fc
        WHERE   fc.topic_id = fl.topic_id
        ) AS replies
FROM    (
        SELECT  topic_id, date AS last_reply
        FROM    wp_pod_tbl_forum fo
        WHERE   id = (
                SELECT  id
                FROM    wp_pod_tbl_forum fp
                WHERE   fp.topic_id = fo.topic_id
                ORDER BY
                        fp.date DESC, fp.id DESC
                LIMIT 1
                )
                AND fo.topic_id <> 0
        ORDER BY
                fo.date DESC, fo.id DESC
        LIMIT 20
        ) fl
JOIN    wp_pod_tbl_forum ft
ON      ft.id = fl.topic_id
UNION ALL
SELECT  id, name, date, 0
FROM    wp_pod_tbl_forum t
WHERE   NOT EXISTS
        (
        SELECT  NULL
        FROM    wp_pod_tbl_forum r
        WHERE   r.topic_id = t.id
        )
        AND t.topic_id = 0
ORDER BY
       last_reply DESC, id DESC
LIMIT  20

这个查询都需要这两个索引才能有效.

如果你的表是InnoDB 和 id是PRIMARY KEY,那么你可以省略indexes上面的id .