使用多个连接优化查询

Question

老实说，我真的认为这是一件简单的事情。但是，我的 MySQL 查询性能非常糟糕，我不知道为什么。

三个 table：个人（10,000,000 行），雇主（50,000 行），person_achievements（250,000 行）

Person (indexes ix_EmpID)
ID int (primary)
Emp_ID int
Full_Name varchar(250)

Employer (indexes ix_ID (ID) and ix_SectID (Sector_ID) and ix_Name) 
ID int (primary)
Sector_ID int
Name varchar (250)

person_achievements (indexes ix_ID (ID) and ix_achievement (achievement))
ID int
Sect_ID
Achievement (varchar 250)

查询：

select p.* 
from person p
join employer c 
    on c.ID = p.Emp_ID
join person_achievements a 
    on a.Sect_ID = c.Sector_ID
where a.Achievement = 'Employee of the Month'

现在我认为我创建的索引会使这个查询执行得更好。然而，它没有。如果我删除 person_achievements table 的连接并保留 where 子句来选择雇主名称（这也是一个索引列），则生成 25,000 行只需要一秒多一点的时间。

我错过了什么？

编辑：添加了人员和雇主主键

编辑 2：

"id","select_type","table","type","possible_keys","key","key_len","ref","rows","Extra"
"1","SIMPLE","a","ref","ix_SectID,ix_Achievement,ix_SectID_Achievement","ix_Achievement","253","const","12654","Using where"
"1","SIMPLE","c","ref","PRIMARY,ix_SectID","ix_SectID","8","db.a.Sect_ID","1",""
"1","SIMPLE","p","ref","Emp_ID","Emp_ID","9","db.c.ID","103","Using where"

Answer 1

这是您的查询：

select p.* 
from person p join
     employer c 
     on c.ID = p.Emp_ID join
     person_achievements a 
     on a.Sect_ID = c.Sector_ID
where a.Achievement = 'Employee of the Month';

最优指标为：person_achievements(Achievement, Sect_id)、employer(sector_id, emp_id)、person(emp_id)。

您可以重新安排内连接，这样您就可以将查询写成：

select p.* 
from person_achievements a join
     employer c 
     on a.Sect_ID = c.Sector_ID join
     person p
     on c.ID = p.Emp_ID
where a.Achievement = 'Employee of the Month';

这应该是 MySQL 选择使用上述索引处理查询的方式。

不过，我不明白的是为什么名为 person_achievements 的 table 没有 person_id 的列（在本例中为 Emp_Id） .数据结构或命名似乎有问题。这种误解可能是问题的核心。如果其中有一个人 ID table，那么您的连接可能会乘以行数，因为它使用了错误的键。

使用多个连接优化查询

Optimizing query with multiple joins

mysql

indexing