使用COUNT的子查询使MYSQL查询变慢

如何解决使用COUNT的子查询使MYSQL查询变慢

| 是的，我不知道为什么，但是此查询需要花费6秒钟以上的时间来执行，索引都已正确设置，如果我分别运行每个查询，它只需不到0.5秒钟的时间就可以很好地工作。这是查询

SELECT c.supplier_id,supplier_name,address1,address2,address3,address4,suppliertype,postcode,contact_name,(SELECT COUNT(*)
    FROM supplier_questions q1
    WHERE c.supplier_id = q1.supplier_id AND q1.incomplete = \'0\') AS questions,IF (active=1,\'Yes\',IF (active=2,\'NCR Only\',\'Inactive\')) AS rated,(SELECT COUNT(*)
    FROM supplier_questions q2
    WHERE c.supplier_id = q2.supplier_id AND q2.reviewed = \'1\') AS reviewed,questapproved,ss.supplier_no AS supplier_no
FROM suppliers c
INNER JOIN supplier_site ss ON c.supplier_id = ss.supplier_id
WHERE c.supplier_id != \'0\' AND ss.site_id = \'2\'
GROUP BY c.supplier_id
ORDER BY c.supplier_name ASC
LIMIT 0,20

Explain查询的结果如下

id  select_type table   type    possible_keys   key key_len ref rows    Extra
1   PRIMARY ss  ref site_id,supplier_id site_id 4   const   1287    Using where; Using temporary; Using filesort
1   PRIMARY c   eq_ref  PRIMARY PRIMARY 4   ss.supplier_id  1   
3   DEPENDENT SUBQUERY  q2  ref supplier_id,reviewed    reviewed    4   const   263 Using where
2   DEPENDENT SUBQUERY  q1  ref supplier_id,incomplete  incomplete  4   const   254 Using where

计数查询存在的原因是因为我需要知道那些表中的行数，这不能在另一个查询中完成，因为结果还需要按这些值进行排序:(

解决方法

作为暗中的一击，这运行得更快吗？（我没有mysql来验证语法，因此可以原谅任何细微的错误，但您可能会明白）

SELECT c.supplier_id,supplier_name,address1,address2,address3,address4,suppliertype,postcode,contact_name,questions,reviewed 
IF (active=1,\'Yes\',IF (active=2,\'NCR Only\',\'Inactive\')) AS rated,questapproved,ss.supplier_no AS supplier_no 
FROM suppliers c 
INNER JOIN supplier_site ss ON c.supplier_id = ss.supplier_id 
inner join 
(SELECT supplier_id,sum(if(incomplete=\'0\',1,0)) as questions,sum(if(incomplete=\'1\',0)) as reviewed FROM supplier_questions q1 group by supplier_id) as tmp
on c.supplier_id = tmp.supplier_id
WHERE c.supplier_id != \'0\' AND ss.site_id = \'2\' 
GROUP BY c.supplier_id 
ORDER BY c.supplier_name ASC LIMIT 0,20

FROM suppliers c
INNER JOIN supplier_site ss ON c.supplier_id = ss.supplier_id
WHERE c.supplier_id != \'0\' AND ss.site_id = \'2\'
GROUP BY c.supplier_id 
ORDER BY c.supplier_name ASC

由于自动生成的主键永远不会等于0（除非出现大的数据库设计错误），所以可以删除c.supplier_id！= \'0 \'子句。 ss.site_id = \'2 \'应该处于JOIN条件以提高可读性。看起来这应该只与每个供应商的表Supplier_site中的一行匹配（如果这是您通常的1-N物-地址关系，即您正在选择每个供应商的第二个地址，则\'2 \'对应于\ '帐单地址\”之类的东西，因此GROUP BY c.supplier_id是无用的。如果GROUP BY实际执行了某项操作，则该查询是错误的，因为“ address \”列（可能来自于vendor_site表）将来自随机行。所以这是简化的FROM（WHERE不在了）：

FROM suppliers c
INNER JOIN supplier_site ss ON 
    (c.supplier_id = ss.supplier_id AND ss.site_id = \'2\')
ORDER BY c.supplier_name ASC

我想您在c.supplier_name上有一个索引，因此查询的这一部分应该非常快。现在尝试以下查询：

SELECT a.*,ss.supplier_no AS supplier_no,IF (active=1,sum( q.incomplete = \'0\') AS questions,sum( q.reviewed = \'1\' ) AS reviewed
FROM
(
    SELECT c.supplier_id,contact_name
    FROM suppliers c
    INNER JOIN supplier_site ss ON 
        (c.supplier_id = ss.supplier_id AND ss.site_id = \'2\')
    ORDER BY c.supplier_name ASC
    LIMIT 0,20
) a
LEFT JOIN supplier_questions q ON (q.supplier_id = c.supplier_id)
GROUP BY c.supplier_id
ORDER BY c.supplier_name;

,如果删除子选择，则结果如下：

SELECT c.supplier_id,COUNT(IF (q1.incomplete = \'0\',\'0\',null)) AS questions,COUNT(IF (q1.reviewed = \'1\',\'1\',null)) AS reviewed,ss.supplier_no AS supplier_no
FROM suppliers c
INNER JOIN supplier_site ss ON c.supplier_id = ss.supplier_id
LEFT OUTER JOIN supplier_questions q1 ON c.supplier_id = q1.supplier_id
WHERE c.supplier_id != \'0\' AND ss.site_id = \'2\'
GROUP BY c.supplier_id
ORDER BY c.supplier_name ASC
LIMIT 0,20

我没有可用的MySQL数据库，因此SQL可能有错误。这个想法是删除子查询并用外部联接替换它们并使用IF仅计算相关行。 ,我将首先尝试通过按供应商预先查询汇总的问题计数并审查一次ONCE来进行重组。然后，加入其余的细节。通过使用STRAIGHT_JOIN关键字，它应该按照显示的顺序进行处理。这将首先进行预汇总，并以THAT为基础加入供应商，然后再加入供应商地点。不需要外部组，因为它无论如何都是基于供应商ID的。但是，如果连接到vendor_sites（您的ss.supplier_no），则意味着供应商具有多个位置。这是否意味着地址和活动状态列源自该表？问题的加入是否应与特定的供应商及其对应的站点位置相关联？此外，由于预查询在Supplier_id！= \'0 \'上具有WHERE子句，因此不需要下游，因为这将成为正常连接其他表的基础，从而将它们从结果集中删除。

SELECT STRAIGHT_JOIN
      PreAggregate.supplier_id,PreAggregate.supplier_name,PreAggregate.Questions,PreAggregate.Reviewed,ss.supplier_no AS supplier_no
   FROM 
      (select 
             s1.Supplier_ID,s1.Supplier_Name,SUM( IF( q1.Incomplete = \'0\',0 )) Questions,SUM( IF( q1.Reviewed = \'1\',0 )) Reviewed
          from 
             suppliers s1
                join supplier_questions q1
                   ON s1.supplier_id = q1.supplier_id
          where
             s1.supplier_id != \'0\'
          group by
             s1.Supplier_ID 
          ORDER BY 
             s1.supplier_name ASC ) PreAggregate

       JOIN suppliers c
          ON PreAggregate.Supplier_ID = c.Supplier_ID

       JOIN supplier_site ss 
          ON PreAggregate.Supplier_ID = ss.supplier_id
          AND ss.Site_ID = \'2\'
  LIMIT 0,20