多個左連接的MySQL計數 - optomization

Question

我有一個查詢通過使用LEFT JOIN和子查詢從多個表中獲取計數。這個想法是讓activites成員曾參加過各種計數

的模式是這樣的：

成員 PK member_id

table1的 PK tbl1_id FK member_id

表2 PK tbl2_id FK member_id

table3 PK tbl 3_id FK member_id

我的查詢看起來是這樣的：

SELECT t1.num1,t2.num2,t3.num3 
FROM member m 
LEFT JOIN 
(
    SELECT member_id,COUNT(*) as num1 
    FROM table1 
    GROUP BY member_id 
) t1 ON t1.member_id = m.member_id 
LEFT JOIN 
(
    SELECT member_id,COUNT(*) as num2 
    FROM table2 
    GROUP BY member_id 
) t2 ON t2.member_id = m.member_id 
LEFT JOIN 
(
    SELECT member_id,COUNT(*) as num3 
    FROM table3 
    GROUP BY member_id 
) t3 ON t3.member_id = m.member_id 
WHERE m.member_id = 27 

其中27是一個測試ID。實際的查詢連接三個以上的表，並且在更改member_id的情況下多次運行查詢。問題是這個查詢運行速度很慢。我得到我需要的信息，但我想知道是否有人可以提出一種方法來優化這一點。任何建議非常感謝。非常感謝。

Answer 1

你應該重構你的查詢。您可以通過重新排序查詢收集數據的方式來做到這一點。怎麼樣？

• 應用WHERE子句第一
• 申請加入上次

這裏是你的原始查詢：

SELECT t1.num1,t2.num2,t3.num3 
FROM member m 
LEFT JOIN 
( 
    SELECT member_id,COUNT(*) as num1 
    FROM table1 
    GROUP BY member_id 
) t1 ON t1.member_id = m.member_id 
LEFT JOIN 
( 
    SELECT member_id,COUNT(*) as num2 
    FROM table2 
    GROUP BY member_id 
) t2 ON t2.member_id = m.member_id 
LEFT JOIN 
( 
    SELECT member_id,COUNT(*) as num3 
    FROM table3 
    GROUP BY member_id 
) t3 ON t3.member_id = m.member_id 
WHERE m.member_id = 27 

這裏是你新的查詢

SELECT 
    IFNULL(t1.num1,0) num1, 
    IFNULL(t1.num2,0) num2, 
    IFNULL(t1.num3,0) num3 
FROM 
(
    SELECT * FROM member m 
    WHERE member_id = 27 
) 
LEFT JOIN 
( 
    SELECT member_id,COUNT(*) as num1 
    FROM table1 
    WHERE member_id = 27 
    GROUP BY member_id 
) t1 ON t1.member_id = m.member_id 
LEFT JOIN 
( 
    SELECT member_id,COUNT(*) as num2 
    FROM table2 
    WHERE member_id = 27 
    GROUP BY member_id 
) t2 ON t2.member_id = m.member_id 
LEFT JOIN 
( 
    SELECT member_id,COUNT(*) as num3 
    FROM table3 
    WHERE member_id = 27 
    GROUP BY member_id 
) t3 ON t3.member_id = m.member_id 
; 

BTW我改爲member m轉換爲SELECT * FROM member m WHERE member_id = 27以防您需要有關成員27的任何信息。我​​還爲每個結果添加了IFNULL函數，以便在count爲NULL的情況下生成0。

你需要絕對確保

• member_id是成員表
• member_id在表1，表2索引的主鍵，表3

試試看！ ！

Answer 2

不知道你的架構和你爲指標做了什麼，一個可能的方式，使這個速度是：

SELECT (select ifnull(count(*),0) from table1 where table1.member_id = m.id) as num1, 
(select ifnull(count(*),0) from table2 where table2.member_id = m.id) as num2, 
(select ifnull(count(*),0) from table3 where table3.member_id = m.id) as num3 
from member m 
WHERE m.member_id = 27 

現在，這是一個稍微高風險的建議，只是因爲我什麼都不知道關於您的數據庫或其他什麼運行，或瓶頸在哪裏。

一般來說，最好是在您的查詢中發佈解釋計劃以獲得更好的答案。

Answer 3

SELECT num1, num2, count(*) as num3 
FROM (
    SELECT member_id, num1, count(*) as num2 
    FROM (
    SELECT member_id, count(*) as num1 
    FROM member 
    LEFT JOIN table1 USING (member_id) 
    WHERE member_id = 27) as m1 
    LEFT JOIN table2 USING (member_id)) as m2 
LEFT JOIN table3 USING (member_id);