Question

首先，例如我有3个表A，B，C。表A与表B有关系，表与表C有关系。我想得到表A中某些字段的SUM，这取决于某些字段来自表C.

表A具有＆gt; 300k行，表B具有> 4k行，表C有~100行

我的查询如下：

SELECT SUM(a.hours) AS total 
FROM table_a a 
LEFT JOIN table_b b 
  ON a.table_b_id = b.id 
LEFT JOIN table_c c
  ON b.table_c_id = c.id 
WHERE a.customer_id = 1
  AND c.title IN ('Title D','Title E')

查询执行时间约为7秒，非常慢。但是如下所述的查询执行时间约为0.0秒。

SELECT a.hours
FROM table_a a 
LEFT JOIN table_b b
  ON a.table_b_id = b.id 
LEFT JOIN table_c c
  ON b.table_c_id = c.id
WHERE a.customer_id = 1
  AND c.title IN ('Title D','Title E')

为什么SUM太慢了？我该怎么办？

Answer 1

将条件移至相关表的ON子句：

SELECT SUM(a.hours) AS total 
FROM table_a a 
LEFT JOIN table_b b 
  ON a.table_b_id = b.id 
LEFT JOIN table_c c
  ON b.table_c_id = c.id 
  AND c.title IN ('Title D','Title E')
WHERE a.customer_id = 1

编辑1 根据@dnoeth评论，我同意，加入inner join时我们可能应该使用table_c：

SELECT SUM(a.hours) AS total 
FROM table_a a 
LEFT JOIN table_b b 
  ON a.table_b_id = b.id 
INNER JOIN table_c c
  ON b.table_c_id = c.id 
  AND c.title IN ('Title D','Title E')
WHERE a.customer_id = 1

Answer 2

在两个地方使用LEFT JOIN并添加此综合索引：

INDEX(customer_id, title)

MySql奇怪的表现

2 个答案: