mysql - 查询不使用索引

Question

谁能告诉我为什么这个查询不使用索引 idx_accessorieslinkid 我创建了索引：

    CREATE INDEX idx_accessorieslinkid ON `accessorieslink` (actionid);

我是索引的新手，并试图了解它们是如何工作的（以及为什么有时它们不这样做！）

sql

 SELECT action.actionid
     FROM
      ACTION 
      LEFT JOIN 
(SELECT 
  SUM(accprice) AS accprice,
  actionid
FROM
  accessorieslink 
GROUP BY actionid) AS accprice 
ON accprice.actionid = action.actionid 
    WHERE action.typeid = '2'
      AND actiondate2 BETWEEN '20130301' AND '20130301' 
    ORDER BY actiondate2 DESC 

解释结果

    id  select_type  table     type         possible_keys                   key key_len ref rows Extra
    1   PRIMARY      action    index_merge  idx_actiontypeid,idx_actiondate idx_actiondate,idx_actiontypeid  4,5 141    Using intersect(idx_actiondate,idx_actiontypeid); Using         where; Using index; Using temporary; Using filesort  
    1   PRIMARY      <derived2>       ALL   7804                    
    2   DERIVED      accessorieslink  ALL   12175                             Using temporary; Using filesort 

Answer 1

派生表不能对物理表使用索引。

试试下面的查询

SELECT  SUM(accprice) AS accprice, accessorieslink.actionid FROM accessorieslink 
LEFT JOIN  `action`  ON accessorieslink.actionid = action.actionid 
GROUP BY accessorieslink.actionid