由optimizer_switch所引起的诡异问题
沃趣科技MySQL高级工程师
一、参数描述
MySQL中不同的版本优化器会有很多新特性，比如MRR、BKA等，其中optimizer_switch这个参数就是控制查询优化器怎样使用这些特性。很多情况下我们会根据自身的需求去设置optimizer_switch满足我们的需求。
前段时间客户的环境中遇到一个奇怪的问题，select count(*)显示返回是有数据，但select * 返回是空结果集，最终的原因就是因为optimizer_switch设置引起了一个让我们难以察觉的BUG。这里和大家分享一下，希望大家在以后的工作如果遇到类似的问题能够轻松应对。
二、案例分析
2.1 环境描述
数据库版本MySQL5.6.35
2.2 SQL语句
select * from (select o.orderid,o.orderdatetime,o.orderstatus,o.price,o.expway,o.paytype,o.fee,o.ordertype,o.realid,mm.account,ms.shopname,mmt.organcode, o.activitype,o.channelcode, ma.activitytag,md.tagtip from mall_order o left join mall_member mm on o.buyerid=mm.memberid left join mall_shop ms on o.salerid=ms.userid left join mall_merchant mmt on mmt.merchantid=o.salerid left join mall_activity ma on o.activityid=ma.actid left join mall_direct_activity md on ma.actid=md.actid where 1=1 and o.orderdatetime >= ‘2017-03-01 01:40:03’ and o.orderdatetime <= ‘2017-03-25 01:40:03’ ) as tab where tab.organcode = ‘805000’ order by orderdatetime desc limit 10;
2.3 分析过程
凌晨4点左右客户打来电话告知数据库查询不到数据，显得非常着急，刻不容缓，我们第一时间赶到了现场，当时的现象是这样的:
select * from (select o.orderid,o.orderdatetime,o.orderstatus,o.price,o.expway,o.paytype,o.fee,o.ordertype,o.realid,mm.account,ms.shopname,mmt.organcode, o.activitype,o.channelcode, ma.activitytag,md.tagtip from mall_order o left join mall_member mm on o.buyerid=mm.memberid left join mall_shop ms on o.salerid=ms.userid left join mall_merchant mmt on mmt.merchantid=o.salerid left join mall_activity ma on o.activityid=ma.actid left join mall_direct_activity md on ma.actid=md.actid where 1=1 and o.orderdatetime >= ‘2017-03-01 01:40:03’ and o.orderdatetime <= ‘2017-03-25 01:40:03’ ) as tab where tab.organcode = ‘805000’ order by orderdatetime desc limit 10;
这条语句查询返回的结果集是空，但是开发人员和我们说数据库中是有数据的，我抱着怀疑的态度尝试着执行了一下:
select * from (select o.orderid,o.orderdatetime,o.orderstatus,o.price,o.expway,o.paytype,o.fee,o.ordertype,o.realid,mm.account,ms.shopname,mmt.organcode, o.activitype,o.channelcode, ma.activitytag,md.tagtip from mall_order o left join mall_member mm on o.buyerid=mm.memberid left join mall_shop ms on o.salerid=ms.userid left join mall_merchant mmt on mmt.merchantid=o.salerid left join mall_activity ma on o.activityid=ma.actid left join mall_direct_activity md on ma.actid=md.actid where 1=1 and o.orderdatetime >= ‘2017-03-01 01:40:03’ and o.orderdatetime <= ‘2017-03-25 01:40:03’ ) as tab where tab.organcode = ‘805000’ order by orderdatetime desc limit 10;
Empty set (0.41 sec)
select count(*) from (select o.orderid,o.orderdatetime,o.orderstatus,o.price,o.expway,o.paytype,o.fee,o.ordertype,o.realid,mm.account,ms.shopname,mmt.organcode, o.activitype,o.channelcode, ma.activitytag,md.tagtip from mall_order o left join mall_member mm on o.buyerid=mm.memberid left join mall_shop ms on o.salerid=ms.userid left join mall_merchant mmt on mmt.merchantid=o.salerid left join mall_activity ma on o.activityid=ma.actid left join mall_direct_activity md on ma.actid=md.actid where 1=1 and o.orderdatetime >= ‘2017-03-01 01:40:03’ and o.orderdatetime <= ‘2017-03-25 01:40:03’ ) as tab where tab.organcode = ‘805000’ order by orderdatetime desc limit 10;
+—————+
| count(*) |
+—————+
| 475 |
+—————+
1 row in set (0.41 sec)
一看结果当时也有点慌了，count(*)显示返回475条记录，但是select *却返回空结果集……
想了一下SQL语句有一层嵌套，我看看里面这个SQL是否有问题，测试后发现内层语句可以正常返回，加上外层语句时就会出现这种情况。询问了应用人员系统刚迁移过来，在原系统没有这种情况，快速连到原系统上执行同样的语句对比一下两边的执行计划:
原系统
explain select * from (select o.orderid,o.orderdatetime,o.orderstatus,o.price,o.expway,o.paytype,o.fee,o.ordertype,o.realid,mm.account,ms.shopname,mmt.organcode, o.activitype,o.channelcode, ma.activitytag,md.tagtip from mall_order o left join mall_member mm on o.buyerid=mm.memberid left join mall_shop ms on o.salerid=ms.userid left join mall_merchant mmt on mmt.merchantid=o.salerid left join mall_activity ma on o.activityid=ma.actid left join mall_direct_activity md on ma.actid=md.actid where 1=1 and o.orderdatetime >= '2017-03-01 01:40:03' and o.orderdatetime <= '2017-03-25 01:40:03' ) as tab where tab.organcode = '805000' order by orderdatetime desc limit 10;
+----+-------------+------------+--------+------------------------+-------------------+---------+-------------------+-------+-----------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------+--------+------------------------+-------------------+---------+-------------------+-------+-----------------------------+
| 1 | PRIMARY |
| ref | | | 153 | const | 10 | Using where; Using filesort | | 2 | DERIVED | o | range | idx_orderdatetime | idx_orderdatetime | 6 | NULL | 46104 | Using index condition |
| 2 | DERIVED | mm | eq_ref | PRIMARY,idx_memberid | PRIMARY | 8 | mall.o.buyerid | 1 | NULL |
| 2 | DERIVED | ms | ref | idx_userid | idx_userid | 9 | mall.o.salerid | 1 | NULL |
| 2 | DERIVED | mmt | eq_ref | PRIMARY,idx_merchantid | PRIMARY | 8 | mall.o.salerid | 1 | NULL |
| 2 | DERIVED | ma | eq_ref | PRIMARY | PRIMARY | 8 | mall.o.activityid | 1 | NULL |
| 2 | DERIVED | md | ref | idx_activityid | idx_activityid | 8 | mall.ma.actid | 1 | NULL |
+----+-------------+------------+--------+------------------------+-------------------+---------+-------------------+-------+-----------------------------+
7 rows in set (0.00 sec)
新系统
explain select * from (select o.orderid,o.orderdatetime,o.orderstatus,o.price,o.expway,o.paytype,o.fee,o.ordertype,o.realid,mm.account,ms.shopname,mmt.organcode, o.activitype,o.channelcode, ma.activitytag,md.tagtip from mall_order o left join mall_member mm on o.buyerid=mm.memberid left join mall_shop ms on o.salerid=ms.userid left join mall_merchant mmt on mmt.merchantid=o.salerid left join mall_activity ma on o.activityid=ma.actid left join mall_direct_activity md on ma.actid=md.actid where 1=1 and o.orderdatetime >= '2017-03-01 01:40:03' and o.orderdatetime <= '2017-03-25 01:40:03' ) as tab where tab.organcode = '805000' order by orderdatetime desc limit 10;
+----+-------------+------------+--------+------------------------+-------------------+---------+-------------------+-------+----------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------+--------+------------------------+-------------------+---------+-------------------+-------+----------------------------------+
| 1 | PRIMARY |
| ref | | | 153 | const | 10 | Using where; Using filesort | | 2 | DERIVED | o | range | idx_orderdatetime | idx_orderdatetime | 6 | NULL | 46104 | Using index condition; Using MRR |
| 2 | DERIVED | mm | eq_ref | PRIMARY,idx_memberid | PRIMARY | 8 | mall.o.buyerid | 1 | NULL |
| 2 | DERIVED | ms | ref | idx_userid | idx_userid | 9 | mall.o.salerid | 1 | NULL |
| 2 | DERIVED | mmt | eq_ref | PRIMARY,idx_merchantid | PRIMARY | 8 | mall.o.salerid | 1 | NULL |
| 2 | DERIVED | ma | eq_ref | PRIMARY | PRIMARY | 8 | mall.o.activityid | 1 | NULL |
| 2 | DERIVED | md | ref | idx_activityid | idx_activityid | 8 | mall.ma.actid | 1 | NULL |
+----+-------------+------------+--------+------------------------+-------------------+---------+-------------------+-------+----------------------------------+
7 rows in set (0.00 sec)
两边的执行计划不同的地方就是新系统使用了MRR，数据库的版本都是5.6.20之后的小版本号没有相差很多，应该不会出现这种情况。
想到了optimizer_switch这个参数可以设置mrr特性，是不是有人对其做了修改，对比了两边optimizer_switch这个参数发现mrr_cost_based这个值设置的不同。快速的将参数设置为一样再次查询:
set optimizer_switch='mrr_cost_based=on';
Query OK, 0 rows affected (0.00 sec)
select * from (select o.orderid,o.orderdatetime,o.orderstatus,o.price,o.expway,o.paytype,o.fee,o.ordertype,o.realid,mm.account,ms.shopname,mmt.organcode, o.activitype,o.channelcode, ma.activitytag,md.tagtip from mall_order o left join mall_member mm on o.buyerid=mm.memberid left join mall_shop ms on o.salerid=ms.userid left join mall_merchant mmt on mmt.merchantid=o.salerid left join mall_activity ma on o.activityid=ma.actid left join mall_direct_activity md on ma.actid=md.actid where 1=1 and o.orderdatetime >= '2017-03-01 01:40:03' and o.orderdatetime <= '2017-03-25 01:40:03' ) as tab where tab.organcode = '805000' order by orderdatetime desc limit 10;
立刻就能够返回数据，一切搞定。
三、总结
mrr_cost_based代表是否使用基于代价的方式去计算使用MRR特性，新的系统中将他设置为off代表不使用基于代价方式而是使用基于规则的，这样设置的原因是考虑到MySQL基于代价的方式比较保守，不能使用到MRR这个特性。本身设置这个参数是没有任何问题，只不过正好遇到mrr_cost_based设置为off时碰到了这么诡异BUG，希望可以帮助到遇到同样问题的朋友们。
来自 “ ITPUB博客 ” ，链接：http://blog.itpub.net/28218939/viewspace-2140197/，如需转载，请注明出处，否则将追究法律责任。
转载于:http://blog.itpub.net/28218939/viewspace-2140197/