Question

我尝试运行此查询，但它挂起了mysql服务器。它击中了大约500万条记录。即使是解释查询也没有及时返回任何内容。

Explain SELECT COUNT(*)   FROM
(SELECT COUNT(DISTINCT Oms_Cart_Id )  Carts,Buyer_Id  
 FROM  Reporting.Order_Details  WHERE Buyer_Id  IN
    ( SELECT Buyer_Id
    FROM Reporting.Order_Details 
    WHERE Payment_Receive_Date>='2015-01-18 00:00:00' AND  
    Payment_Receive_Date<='2015-01-24 23:59:59'
    GROUP BY  Buyer_Id )  
 AND Payment_Receive_Date>='2014-09-01 00:00:00'   
 AND  Payment_Receive_Date<='2015-01-24 23:59:59'
 GROUP BY 2) X
 WHERE Carts>1
 LIMIT 0, 1000 ;

Answer 1

您向我们展示的查询应该是＆＃34;点击＆＃34;除非reporting.order_details是联合表，否则大约4行。假设不是这种情况，或者您的查询被其他东西阻止，或者您的服务器出现了问题。 Mysqladmin进程列表会告诉你前者是否是这种情况。您的系统日志应显示磁盘的任何问题。如果不是这种情况，那么您的数据文件可能已损坏。

Answer 2

（是的，大脑受伤）

摆脱无用的LIMIT，只返回一行。
将外围的WHERE变成内在的HAVING（没有充分理由）
将IN（SELECT ...）转换为JOIN。我开始做一个自我加入，但是GROUP BY说或许子查询只提供了几个Buyer_Ids。所以，我做了JOIN（SELECT ...）

结果：

SELECT  COUNT(*)
    FROM  
      ( SELECT  COUNT(DISTINCT a.Oms_Cart_Id ) Carts,
                a.Buyer_Id
            FROM  Reporting.Order_Details a
            JOIN  
              ( SELECT  Buyer_Id
                    FROM  Reporting.Order_Details
                    WHERE  Payment_Receive_Date>='2015-01-18 00:00:00'
                      AND  Payment_Receive_Date<='2015-01-24 23:59:59'
                    GROUP BY  Buyer_Id
              ) AS b USING(Buyer_Id)
            WHERE  a.Payment_Receive_Date>='2014-09-01 00:00:00'
              AND  a.Payment_Receive_Date<='2015-01-24 23:59:59'
            GROUP BY  2
            HAVING  Carts > 1 
      ) X ;

Order_Details将受益于这两个：

INDEX(Buyer_Id) - 对于JOIN。（如果您已经使用Buyer_Id开始索引，请不要添加此索引。）

INDEX(Payment_Receive_Date, Buyer_Id) - 用于子查询

如果你真的只做一个COUNT（*）：

SELECT  COUNT(DISTINCT Buyer_Id)
    FROM  Reporting.Order_Details a
    JOIN  
      ( SELECT  Buyer_Id
            FROM  Reporting.Order_Details
            WHERE  Payment_Receive_Date>='2015-01-18 00:00:00'
              AND  Payment_Receive_Date<='2015-01-24 23:59:59'
            GROUP BY  Buyer_Id
      ) AS b USING(Buyer_Id)
    WHERE  Payment_Receive_Date>='2014-09-01 00:00:00'
      AND  Payment_Receive_Date<='2015-01-24 23:59:59'
    GROUP BY  2
    HAVING  COUNT(DISTINCT a.Oms_Cart_Id) > 1

为什么这个SQL查询需要永远？

2 个答案: