paip.sql索引优化----join 代替子查询法

paip.sql索引优化----join 代替子查询法

作者Attilax ，  EMAIL:1466519819@qq.com

来源：attilax的专栏

地址：http://blog.csdn.net/attilax

 select SQL_NO_CACHE  *,(

    SELECT

     LEFT (fecye, 1)

    FROM

     cyefenbyao   force index(i_hezi) 

    WHERE

     cyefenbyao.hezi = rit

  limit 1

   ) AS rit_cye

  FROM

   (

    select  

    max( gaopinzi.HEZI) hezi,

     LEFT (hezi, 1) AS lft,

     RIGHT (hezi, 1) AS rit

    FROM

     gaopinzi 

    WHERE

     LENGTH(hezi) = 6

 

      group by HEZI

        limit 30000

   ) AS t2 

要1S,改成join  0.5S..

select SQL_NO_CACHE 

t2.*  ,LEFT (fecye, 1) as t1

FROM

 (

  SELECT

   max(gaopinzi.HEZI) hezi,

   LEFT (hezi, 1) AS lft,

   RIGHT (hezi, 1) AS rit

  FROM

   gaopinzi

  WHERE

   LENGTH(hezi) = 6

  GROUP BY

   HEZI

  LIMIT 30000

 ) AS t2

left join  cyefenbyao

on cyefenbyao.hezi = t2.rit