Question

这两个mysql查询产生完全相同的结果，但查询A是一个简单的联合，它有where postType子句嵌入在单个查询中，而查询B具有相同的where子句应用于虚拟表的外部选择，这是单个查询结果的联合。我担心来自查询B的虚拟表可能变得太大而没有充分的理由如果有很多行但是我有点困惑因为查询A的工作顺序如何;是否也不必为了排序结果而制作虚拟表或类似的东西。所有这些都可能取决于工会的工作顺序如何？如果一个联盟的命令也在制作临时表;那么查询A几乎等同于资源中的查询B（与查询A相比，我们在系统中实现查询B要容易得多）？请以任何可能的方式提供指导/建议。

查询A

 SELECT `t1`.*, `t2`.ignorableBalancingColumnFromQuery1
             FROM `t1` INNER JOIN `t2` ON
             `t1`.websiteID= `t2`.ownerID
             AND `t1`.authorID= `t2`.authorID
             AND `t1`.authorID=1559 AND `t1`.postType="simplePost"
 UNION
             SELECT `t1`.*, 111 AS ignorableBalancingColumnFromQuery1
              FROM `t1` where websiteID=1559 AND postType="simplePost" 
 ORDER BY postID limit 0,50

查询B

 Select * from (
 SELECT `t1`.*,`t2`.ignorableBalancingColumnFromQuery1
             FROM `t1` INNER JOIN `t2` ON
              `t1`.websiteID= `t2`.ownerID
               AND `t1`.authorID= `t2`.authorID
             AND `t1`.authorID=1559
 UNION
             SELECT `t1`.*, 111 AS ignorableBalancingColumnFromQuery1
              FROM `t1` where websiteID=1559
 )
 As sigma  where postType="simplePost" ORDER BY postID limit 0,50

解析查询

id    type                table           type    possible_keys   keys            key_len ref         rows    Extra
1     PRIMARY             t2              ref     userID          userID          4       const       1       
1     PRIMARY             t1              ref     authorID        authorID        4       const       2       Usingwhere
2     UNION               t1              ref     websiteID       websiteID       4       const       9       Usingwhere
NULL  UNIONRESULT         <union1,2>      ALL     NULL            NULL            NULL    NULL        NULL    Usingfilesort

解析查询B

id    select_type     table       type    possible_keys   key         key_len     ref     rows    Extra
1     PRIMARY         <derived2>  ALL     NULL            NULL        NULL        NULL    10      Using where; Using filesort
2     DERIVED         t2          ref     userID          userID          4               1   
2     DERIVED         t1          ref     authorID        authorID        4               2       Using where
3     UNION           t1          ref     websiteID       websiteID       4               9   
NULL  UNION RESULT    <union2,3>  ALL     NULL            NULL        NULL        NULL    NULL

涉及union和virtual / derived表的高效查询

0 个答案: