MySQL 查询优化 - 内部查询

Question

这是整个查询...

SELECT s.*, (SELECT url FROM show_medias WHERE show_id = s.id AND is_primary = 1) AS media_url
FROM (shows As s)
WHERE `s`.`id` IN (
 SELECT DISTINCT st.show_id
 FROM show_time_schedules AS sts
 LEFT JOIN show_times AS st ON st.id = sts.show_time_id
 WHERE sts.schedule_date BETWEEN CAST('2012-01-10' AS date) AND CAST('2012-01-14' AS date)
 )
AND `s`.`is_active` = 1
ORDER BY s.name asc 

如果...

SELECT url FROM show_medias WHERE show_id = s.id AND is_primary = 1
(0.0004 sec)

还有……

 SELECT DISTINCT st.show_id
 FROM show_time_schedules AS sts
 LEFT JOIN show_times AS st ON st.id = sts.show_time_id
 WHERE sts.schedule_date BETWEEN CAST('2012-01-10' AS date) AND CAST('2012-01-14' AS date)
(0.0061 sec)

是否有明显的原因......

SELECT s.*, (inner query 1) AS media_url
FROM (shows As s)
WHERE `s`.`id` IN ( inner query 2 )
AND `s`.`is_active` = 1
ORDER BY s.name asc

需要 5.7245 秒吗？

解释扩展

id  select_type         table       type    possible_keys   key     key_len ref                     rows    filtered    Extra
1   PRIMARY             s           ALL     NULL            NULL    NULL    NULL                    151     100.00      Using where; Using filesort
3   DEPENDENT SUBQUERY  sts         ALL     NULL            NULL    NULL    NULL                    26290   100.00      Using where; Using temporary
3   DEPENDENT SUBQUERY  st          eq_ref  PRIMARY         PRIMARY 4       bvcdb.sts.show_time_id  1       100.00      Using where
2   DEPENDENT SUBQUERY  show_medias ALL     NULL            NULL    NULL    NULL                    159     100.00      Using where

Answer 1

您可以随时使用 EXPLAIN or EXPLAIN EXTENDED查看 MySql 对查询做了什么

您也可以用稍微不同的方式编写查询，您是否尝试过以下方法？

SELECT        s.*, 
              sm.url AS media_url 
FROM          shows AS s
INNER JOIN    show_medias AS sm ON s.id = SM.show_id
WHERE `s`.`id` IN ( 
                        SELECT DISTINCT st.show_id 
                        FROM show_time_schedules AS sts 
                        LEFT JOIN show_times AS st ON st.id = sts.show_time_id 
                        WHERE sts.schedule_date BETWEEN CAST('2012-01-10' AS date) AND CAST('2012-01-14' AS date) 
                        ) 
AND            `s`.`is_active` = 1 
AND            sm.is_primary = 1
ORDER BY       s.name asc 

看看它的效果会很有趣。我希望它会更快，因为目前，我认为 MySql 将为您的每个节目运行内部查询 1(这样一个查询将运行多次。连接应该更有效。)

如果您想要在 show_medias 中没有一行的所有节目，请将 INNER JOIN 替换为 LEFT JOIN。

编辑:

我很快就会看一下您的 EXPLAIN EXTENDED，我也想知道您是否想尝试以下内容；它删除所有子查询:

SELECT        DISTINCT s.*,  
                       sm.url AS media_url  
FROM                   shows AS s 
INNER JOIN             show_medias AS sm ON s.id = SM.show_id
INNER JOIN             show_times AS st ON (s.id = st.show_id)
RIGHT JOIN             show_time_schedules AS sts ON (st.id = sts.show_time_id)

WHERE                  `s`.`is_active` = 1  
AND                    sm.is_primary = 1 
AND                    sts.schedule_date BETWEEN CAST('2012-01-10' AS date) AND CAST('2012-01-14' AS date)  
ORDER BY               s.name asc 

(如果能看到关于这些的 EXPLAIN EXTENDED 也很好 - 您可以将其添加到对此的评论中)。

进一步编辑:

关于您的 EXPLAIN EXTENDED ( a good start on how to read these is here )

USING FILESORT 和 USING TEMPORARY 都是关键指标。希望我推荐的第二个查询应该删除任何临时表(在子查询中)。然后尝试关闭 ORDER BY 以查看这是否有所不同(我们可以将其添加到目前的调查结果中:-)

我还可以看到查询可能遗漏了很多索引查找；您所有的 id 列都是索引匹配的主要候选者(通常是 index caveats )。我也会尝试添加这些索引，然后再次运行 EXPLAIN EXTENDED 以查看现在有什么区别(我们已经从您上面的评论中知道了编辑!)