Question

在下面的查询中，我要ORDER BY RAND（）c表。当我将ORDER BY RAND（）放入JOIN时，由于ORDER BY在GROUP BY之前运行，因此查询需要执行5秒钟以上。

UPDATE  `backlinks` as a
    JOIN (
        SELECT  b.`id` as bid
            FROM  `backlinks` b
            WHERE  b.`googlebot_id` IS NULL
              AND  b.`used_time` IS NULL
              AND  b.`campaign_id` IN (
                SELECT  `id`  FROM  `campaigns`  WHERE  `status`=true
                                      )
            GROUP BY  b.`campaign_id` 
         ) AS c  ON a.id = c.bid
    SET a.`crawler_id` = 'test'
    limit  1;

Answer 1

为什么要通过分组汇总功能使用分组方式
如果您只想为每个b设置一行。campaign_id使用一些聚合函数可以避免其他列值的不可预测的结果以及db最新版本的错误

适当的聚合函数可以避免订单数量为1并限制为

为了提高性能，您可以避免为子查询使用IN子句，而使用内部联接可以产生相同的结果，但速度更快

UPDATE `backlinks` as a
JOIN(
    SELECT min(b.`id`) as bid
    FROM `backlinks` b
    INNER JOIN (
          SELECT `id` 
          FROM `campaigns` 
          WHERE `status`=true
    ) t1 on t1.id = b.`campaign_id` 
    WHERE b.`googlebot_id` IS NULL 
    AND b.`used_time` IS NULL 
    GROUP BY b.`campaign_id`
) AS c ON a.id = c.bid
SET a.`crawler_id` = 'test'
limit 1;

无论如何，如果您使用的是mysql prevoius版本，则5.7可以使用不具有聚集功能的分组依据..和..进行排序，但是..两者都会影响性能

 UPDATE `backlinks` as a
JOIN(
    SELECT b.`id` as bid
    FROM `backlinks` b
    INNER JOIN (
          SELECT `id` 
          FROM `campaigns` 
          WHERE `status`=true
    ) t1 on t1.id = b.`campaign_id` 
    WHERE b.`googlebot_id` IS NULL 
    AND b.`used_time` IS NULL 
    GROUP BY b.`campaign_id`

) AS c ON a.id = c.bid

SET a.`crawler_id` = 'test'

limit 1;

提高性能的独特方式与使用join而不是IN子句以及表反向链接列campaign_id上的正确索引有关

您可以尝试在子查询外部但在适当的外部子查询内部使用rand和limit限制，并加入更新结果

UPDATE `backlinks` as a 
INNER JOIN  (
  select  a1.id 
  from backlinks as a1
  INNER JOIN (
     SELECT b.`id` as bid
      FROM `backlinks` b
      INNER JOIN (
            SELECT `id` 
            FROM `campaigns` 
            WHERE `status`=true
      ) t1 on t1.id = b.`campaign_id` 
      WHERE b.`googlebot_id` IS NULL 
      AND b.`used_time` IS NULL 
      GROUP BY b.`campaign_id`
  )  AS c ON a1.id = c.bid 
  ORDER BY rand()  
  limit 1  
) t on t.id = a.id

如何按临时表排序

1 个答案: