通過多個連接優化查詢

老實說，我真的認爲這是一件簡單的事情。然而，我的MySQL查詢性能是可怕的，我不知道爲什麼。通過多個連接優化查詢

三個表：人（10,000,000行），僱主（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)

查詢：

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'

現在，我想我創建的索引將導致這個查詢更好地執行LOT。但是，它沒有。如果刪除person_achievements表的連接並保留where子句以選擇僱主名稱（也是索引列），則需要超過一秒鐘才能生成25,000行。

我錯過了什麼？

編輯：添加人員和僱主的主鍵

編輯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"

來源

2015-01-09 user2124871

後解釋 - 和正確的DDLs不會傷害！ – Strawberry

另外，嘗試刪除成就指數。相反，把它放在（Sect_ID，Achievement） – Strawberry

大概所有叫做ID的都是PRIMARY KEY？ – Strawberry

這是您的查詢：

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';

的最佳指標是：person_achievements(Achievement, Sect_id)，employer(sector_id, emp_id)和person(emp_id)。

您可以重新安排內部聯接，這樣你就可以編寫查詢爲：

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';

這應該是MySQL的選擇如何處理與上述指標的查詢。

但我不明白，爲什麼一個名爲person_achievements的表格沒有person_id（本例中爲Emp_Id）的列。數據結構或命名似乎有些不妥。這種誤解可能是問題的核心。如果該表中存在人員ID，那麼您的連接可能會乘以行數，因爲它使用了錯誤的鍵。

來源

2015-01-09 01:01:42

通過多個連接優化查詢

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