Question

我有一个带有多个左连接的Distinct Select语句，当我的where子句很大时，它表现不佳。以下是我的陈述

SELECT  DISTINCT u.*, ri.id as reg_id, d.id as dist_id
    FROM  users u
    LEFT JOIN  earned_points ep ON u.id = ep.user_id
    LEFT JOIN  distributors d ON d.id = ep.distributor_id
      OR  d.id = u.distributor_id
      OR  d.id = u.additional_distributor_id
    LEFT JOIN  registration_items_users riu ON u.id = riu.user_id
      AND  riu.distributor_id = d.id
      AND  riu.registration_item_id = 21
    LEFT JOIN  registration_items ri ON riu.registration_item_id = ri.id
    WHERE  d.id IN (201,281,321,631,901,971,1211,1601,1611,1621,
               1631,1641,1651,1661,1671,1681,1691,1701,1711,1721,1731,
               1741,1751,1761,1771,1781,2281,2291,2401,2781,2801,2931 );

此查询的说明如下：

此查询大约需要4秒钟才能完成。如果我将地方减少到一个id，那么速度可以达到约170毫秒。

欢迎任何有关如何更快地进行此查询的建议。

谢谢

修改

我能够提出一个基于Rick James（接受的答案）建议的解决方案。使用Union并摆脱Left Joins和Distinct就可以了。与上面的4秒版本相比，这个新查询大约需要200毫秒。

(SELECT  u.*, 
   (SELECT riu.registration_item_id 
       FROM registration_items_users riu 
       WHERE riu.user_id = u.id 
           AND riu.distributor_id = d.id 
           AND riu.registration_item_id = 21) as reg_id,
   d.id as dist_id
   FROM users u
   JOIN earned_points ep ON u.id = ep.user_id
   JOIN distributors d ON d.id = ep.distributor_id
       WHERE d.id IN (201,281,321,631,901,971,1211,1601,1611,1621,
            1631,1641,1651,1661,1671,1681,1691,1701,1711,1721,1731,
            1741,1751,1761,1771,1781,2281,2291,2401,2781,2801,2931))
   UNION
(SELECT  u.*, 
   (SELECT riu.registration_item_id 
       FROM registration_items_users riu 
       WHERE riu.user_id = u.id 
           AND riu.distributor_id = d.id 
           AND riu.registration_item_id = 21) as reg_id,
   d.id as dist_id
   FROM users u
   JOIN distributors d ON d.id = u.distributor_id
       WHERE d.id IN (201,281,321,631,901,971,1211,1601,1611,1621,
            1631,1641,1651,1661,1671,1681,1691,1701,1711,1721,1731,
            1741,1751,1761,1771,1781,2281,2291,2401,2781,2801,2931))
   UNION
(SELECT  u.*, 
   (SELECT riu.registration_item_id 
       FROM registration_items_users riu 
       WHERE riu.user_id = u.id 
           AND riu.distributor_id = d.id 
           AND riu.registration_item_id = 21) as reg_id,
   d.id as dist_id
   FROM users u
   JOIN distributors d ON d.id = u.additional_distributor_id
       WHERE d.id IN (201,281,321,631,901,971,1211,1601,1611,1621,
            1631,1641,1651,1661,1671,1681,1691,1701,1711,1721,1731,
            1741,1751,1761,1771,1781,2281,2291,2401,2781,2801,2931))

Answer 1

在EXPLAIN中，查看u行。它正在进行大约6974行的“表扫描”。

除非“正确”表格是可选的，否则摆脱LEFT。

将OR变为UNION;这就是索引失败的地方。（UNION ALL比UNION DISTINCT快;选择有意义的。）

假设可以移除LEFTs，并假设DISTINCT可以从SELECT移至UNION：

SELECT  u.*, ri.id as reg_id, d.id as dist_id
    FROM  users u
    JOIN  earned_points ep ON u.id = ep.user_id  -- ep needed only for this
    JOIN  distributors d ON d.id = ep.distributor_id  -- This one line differs
    JOIN  registration_items_users riu ON u.id = riu.user_id
      AND  riu.distributor_id = d.id
      AND  riu.registration_item_id = 21
    JOIN  registration_items ri ON riu.registration_item_id = ri.id
    WHERE  d.id IN (201,281,321,631,901,971,1211,1601,1611,1621,
                1631,1641,1651,1661,1671,1681,1691,1701,1711,1721,1731,
                1741,1751,1761,1771,1781,2281,2291,2401,2781,2801,2931 
                   )
    UNION  DISTINCT 
SELECT  u.*, ri.id as reg_id, d.id as dist_id
    FROM  users u
    JOIN  distributors d ON d.id = u.distributor_id
    JOIN  registration_items_users riu ON u.id = riu.user_id
      AND  riu.distributor_id = d.id
      AND  riu.registration_item_id = 21
    JOIN  registration_items ri ON riu.registration_item_id = ri.id
    WHERE  d.id IN (201,281,321,631,901,971,1211,1601,1611,1621,
                1631,1641,1651,1661,1671,1681,1691,1701,1711,1721,1731,
                1741,1751,1761,1771,1781,2281,2291,2401,2781,2801,2931 
                   )
    UNION  DISTINCT 
SELECT  u.*, ri.id as reg_id, d.id as dist_id
    FROM  users u
    JOIN  distributors d ON d.id = u.additional_distributor_id
    JOIN  registration_items_users riu ON u.id = riu.user_id
      AND  riu.distributor_id = d.id
      AND  riu.registration_item_id = 21
    JOIN  registration_items ri ON riu.registration_item_id = ri.id
    WHERE  d.id IN (201,281,321,631,901,971,1211,1601,1611,1621,
                1631,1641,1651,1661,1671,1681,1691,1701,1711,1721,1731,
                1741,1751,1761,1771,1781,2281,2291,2401,2781,2801,2931 
                   ) ;

跨列传播数组通常是一个坏主意。这似乎是distributors发生的事情。这种混乱可能是这样的结果。

修改

更好的方法是将ri和rui内容从选择中拉出来并将其转换为子查询。这是要点;我没有精力写下这一切：

SELECT x.*, ( SELECT ... ri and rui stuff ... ) AS reg_id FROM ( -- from above, less the ri and rui stuff: SELECT ... UNION DISTINCT SELECT ... UNION DISTINCT SELECT ... ) AS x;

MySQL明显表现不佳

1 个答案: