mysql的一个bug Block Nested Loop

2019独角兽企业重金招聘Python工程师标准>>>

版本 5.7.20-19-log

mysql 官网bug https://bugs.mysql.com/bug.php?id=69721  

主库和从库配置一样，但是执行计划不一样。

sql

SELECT ORD.*, INFO.NAME AS SHOPNAME, INFO.CONTACT_PHONE AS SHOPPHONE, IF(SS.ID IS NULL, 0, 0)AS REFUNDSTATUS
FROM SHOP_ORDER_INFO ORD
LEFT JOIN SHOP_INFO INFO ON INFO.ID=ORD.SHOP_ID
LEFT JOIN(SELECT ID, ORDER_ID FROM SHOP_ORDER_AFTER_SALES
WHERE STATUS='REFUND_SUCCESS' GROUP BY ORDER_ID)SS ON ORD.ID=SS.ORDER_ID
WHERE ORD.DEL_FLAG='0'
ORDER BY ORD.CREATE_DATE DESC
limit 0,10

主库执行计划

1 PRIMARY ORD ALL 36547 10 Using where; Using temporary; Using filesort
1 PRIMARY INFO eq_ref PRIMARY PRIMARY 194 fengdai_shop.ORD.shop_id 1 100
1 PRIMARY ALL 3 100 Using where; Using join buffer (Block Nested Loop)
2 DERIVED SHOP_ORDER_AFTER_SALES index idx_order_id idx_order_id 195 36 10 Using where

从库执行计划

1 PRIMARY ORD ALL 43970 10 Using where; Using filesort
1 PRIMARY INFO eq_ref PRIMARY PRIMARY 194 fengdai_shop.ORD.shop_id 1 100
1 PRIMARY ref 195 fengdai_shop.ORD.id 3 100
2 DERIVED SHOP_ORDER_AFTER_SALES index idx_order_id idx_order_id 195 368 10 Using where

主库比从库慢了5倍

执行器的选择有问题 明明派生表生成索引速度更快，但是主库选择了Block Nested Loop算法。

解决方法：

1 sql换一种方式

2 当前会话把这种算法关掉

SET SESSION optimizer_switch ='block_nested_loop=off';

select @@optimizer_switch;

 

 

 

转载于:https://my.oschina.net/zhuzhenyu/blog/2873684