optimizer_switch引起的诡异问题

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，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是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BUG。这里和大家分享下防止大家也遇到同样的坑。

案例分析

环境描述

数据库版本MySQL5.6.35

SQL语句

1. 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 where1=1and 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;

分析过程

当时凌晨4点左右客户打来电话告知数据库查询不到数据了非常着急，我们赶到了现场，当时的现象是这样的:

1. 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 where1=1and 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，我当时不相信就执行了一下:

1. 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 where1=1and 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. Emptyset(0.41 sec)
5. 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 where1=1and 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;
6. +----------+
7. | count(*)|
8. +----------+
9. |475|
10. +----------+
11. 1 row inset(0.41 sec)

当时也是比较慌张了，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

1. 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 where1=1and 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. | id | select_type | table | type | possible_keys | key | key_len |ref| rows |Extra|
4. +----+-------------+------------+--------+------------------------+-------------------+---------+-------------------+-------+-----------------------------+
5. |1| PRIMARY |<derived2>|ref|<auto_key0>|<auto_key0>|153|const|10|Usingwhere;Using filesort |
6. |2| DERIVED | o | range | idx_orderdatetime | idx_orderdatetime |6| NULL |46104|Using index condition |
7. |2| DERIVED | mm | eq_ref | PRIMARY,idx_memberid | PRIMARY |8| mall.o.buyerid |1| NULL |
8. |2| DERIVED | ms |ref| idx_userid | idx_userid |9| mall.o.salerid |1| NULL |
9. |2| DERIVED | mmt | eq_ref | PRIMARY,idx_merchantid | PRIMARY |8| mall.o.salerid |1| NULL |
10. |2| DERIVED | ma | eq_ref | PRIMARY | PRIMARY |8| mall.o.activityid |1| NULL |
11. |2| DERIVED | md |ref| idx_activityid | idx_activityid |8| mall.ma.actid |1| NULL |
12. +----+-------------+------------+--------+------------------------+-------------------+---------+-------------------+-------+-----------------------------+
13. 7 rows inset(0.00 sec)

新系统

1. 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 where1=1and 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. | id | select_type | table | type | possible_keys | key | key_len |ref| rows |Extra|
4. +----+-------------+------------+--------+------------------------+-------------------+---------+-------------------+-------+----------------------------------+
5. |1| PRIMARY |<derived2>|ref|<auto_key0>|<auto_key0>|153|const|10|Usingwhere;Using filesort |
6. |2| DERIVED | o | range | idx_orderdatetime | idx_orderdatetime |6| NULL |46104|Using index condition;Using MRR |
7. |2| DERIVED | mm | eq_ref | PRIMARY,idx_memberid | PRIMARY |8| mall.o.buyerid |1| NULL |
8. |2| DERIVED | ms |ref| idx_userid | idx_userid |9| mall.o.salerid |1| NULL |
9. |2| DERIVED | mmt | eq_ref | PRIMARY,idx_merchantid | PRIMARY |8| mall.o.salerid |1| NULL |
10. |2| DERIVED | ma | eq_ref | PRIMARY | PRIMARY |8| mall.o.activityid |1| NULL |
11. |2| DERIVED | md |ref| idx_activityid | idx_activityid |8| mall.ma.actid |1| NULL |
12. +----+-------------+------------+--------+------------------------+-------------------+---------+-------------------+-------+----------------------------------+
13. 7 rows inset(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，是不是有人修改了他，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:

1. set optimizer_switch='mrr_cost_based=on';
2. Query OK,0 rows affected (0.00 sec)
5. 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 where1=1and 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;

立刻就返回数据了。

小结

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，只不过是正好将遇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。

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，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是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BUG。这里和大家分享下防止大家也遇到同样的坑。

案例分析

环境描述

数据库版本MySQL5.6.35

SQL语句

1. 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 where1=1and 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;

分析过程

当时凌晨4点左右客户打来电话告知数据库查询不到数据了非常着急，我们赶到了现场，当时的现象是这样的:

1. 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 where1=1and 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，我当时不相信就执行了一下:

1. 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 where1=1and 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. Emptyset(0.41 sec)
5. 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 where1=1and 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;
6. +----------+
7. | count(*)|
8. +----------+
9. |475|
10. +----------+
11. 1 row inset(0.41 sec)

当时也是比较慌张了，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

1. 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 where1=1and 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. | id | select_type | table | type | possible_keys | key | key_len |ref| rows |Extra|
4. +----+-------------+------------+--------+------------------------+-------------------+---------+-------------------+-------+-----------------------------+
5. |1| PRIMARY |<derived2>|ref|<auto_key0>|<auto_key0>|153|const|10|Usingwhere;Using filesort |
6. |2| DERIVED | o | range | idx_orderdatetime | idx_orderdatetime |6| NULL |46104|Using index condition |
7. |2| DERIVED | mm | eq_ref | PRIMARY,idx_memberid | PRIMARY |8| mall.o.buyerid |1| NULL |
8. |2| DERIVED | ms |ref| idx_userid | idx_userid |9| mall.o.salerid |1| NULL |
9. |2| DERIVED | mmt | eq_ref | PRIMARY,idx_merchantid | PRIMARY |8| mall.o.salerid |1| NULL |
10. |2| DERIVED | ma | eq_ref | PRIMARY | PRIMARY |8| mall.o.activityid |1| NULL |
11. |2| DERIVED | md |ref| idx_activityid | idx_activityid |8| mall.ma.actid |1| NULL |
12. +----+-------------+------------+--------+------------------------+-------------------+---------+-------------------+-------+-----------------------------+
13. 7 rows inset(0.00 sec)

新系统

1. 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 where1=1and 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. | id | select_type | table | type | possible_keys | key | key_len |ref| rows |Extra|
4. +----+-------------+------------+--------+------------------------+-------------------+---------+-------------------+-------+----------------------------------+
5. |1| PRIMARY |<derived2>|ref|<auto_key0>|<auto_key0>|153|const|10|Usingwhere;Using filesort |
6. |2| DERIVED | o | range | idx_orderdatetime | idx_orderdatetime |6| NULL |46104|Using index condition;Using MRR |
7. |2| DERIVED | mm | eq_ref | PRIMARY,idx_memberid | PRIMARY |8| mall.o.buyerid |1| NULL |
8. |2| DERIVED | ms |ref| idx_userid | idx_userid |9| mall.o.salerid |1| NULL |
9. |2| DERIVED | mmt | eq_ref | PRIMARY,idx_merchantid | PRIMARY |8| mall.o.salerid |1| NULL |
10. |2| DERIVED | ma | eq_ref | PRIMARY | PRIMARY |8| mall.o.activityid |1| NULL |
11. |2| DERIVED | md |ref| idx_activityid | idx_activityid |8| mall.ma.actid |1| NULL |
12. +----+-------------+------------+--------+------------------------+-------------------+---------+-------------------+-------+----------------------------------+
13. 7 rows inset(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，是不是有人修改了他，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:

1. set optimizer_switch='mrr_cost_based=on';
2. Query OK,0 rows affected (0.00 sec)
5. 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 where1=1and 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;.

立刻就返回数据了。

小结

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，只不过是正好将遇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。

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，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是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BUG。这里和大家分享下防止大家也遇到同样的坑。

案例分析

环境描述

数据库版本MySQL5.6.35

SQL语句

1. 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 where1=1and 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;

分析过程

当时凌晨4点左右客户打来电话告知数据库查询不到数据了非常着急，我们赶到了现场，当时的现象是这样的:

1. 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 where1=1and 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，我当时不相信就执行了一下:

1. 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 where1=1and 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. Emptyset(0.41 sec)
5. 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 where1=1and 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;
6. +----------+
7. | count(*)|
8. +----------+
9. |475|
10. +----------+
11. 1 row inset(0.41 sec)

当时也是比较慌张了，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

1. 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 where1=1and 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. | id | select_type | table | type | possible_keys | key | key_len |ref| rows |Extra|
4. +----+-------------+------------+--------+------------------------+-------------------+---------+-------------------+-------+-----------------------------+
5. |1| PRIMARY |<derived2>|ref|<auto_key0>|<auto_key0>|153|const|10|Usingwhere;Using filesort |
6. |2| DERIVED | o | range | idx_orderdatetime | idx_orderdatetime |6| NULL |46104|Using index condition |
7. |2| DERIVED | mm | eq_ref | PRIMARY,idx_memberid | PRIMARY |8| mall.o.buyerid |1| NULL |
8. |2| DERIVED | ms |ref| idx_userid | idx_userid |9| mall.o.salerid |1| NULL |
9. |2| DERIVED | mmt | eq_ref | PRIMARY,idx_merchantid | PRIMARY |8| mall.o.salerid |1| NULL |
10. |2| DERIVED | ma | eq_ref | PRIMARY | PRIMARY |8| mall.o.activityid |1| NULL |
11. |2| DERIVED | md |ref| idx_activityid | idx_activityid |8| mall.ma.actid |1| NULL |
12. +----+-------------+------------+--------+------------------------+-------------------+---------+-------------------+-------+-----------------------------+
13. 7 rows inset(0.00 sec)

新系统

1. 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 where1=1and 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. | id | select_type | table | type | possible_keys | key | key_len |ref| rows |Extra|
4. +----+-------------+------------+--------+------------------------+-------------------+---------+-------------------+-------+----------------------------------+
5. |1| PRIMARY |<derived2>|ref|<auto_key0>|<auto_key0>|153|const|10|Usingwhere;Using filesort |
6. |2| DERIVED | o | range | idx_orderdatetime | idx_orderdatetime |6| NULL |46104|Using index condition;Using MRR |
7. |2| DERIVED | mm | eq_ref | PRIMARY,idx_memberid | PRIMARY |8| mall.o.buyerid |1| NULL |
8. |2| DERIVED | ms |ref| idx_userid | idx_userid |9| mall.o.salerid |1| NULL |
9. |2| DERIVED | mmt | eq_ref | PRIMARY,idx_merchantid | PRIMARY |8| mall.o.salerid |1| NULL |
10. |2| DERIVED | ma | eq_ref | PRIMARY | PRIMARY |8| mall.o.activityid |1| NULL |
11. |2| DERIVED | md |ref| idx_activityid | idx_activityid |8| mall.ma.actid |1| NULL |
12. +----+-------------+------------+--------+------------------------+-------------------+---------+-------------------+-------+----------------------------------+
13. 7 rows inset(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，是不是有人修改了他，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:

1. set optimizer_switch='mrr_cost_based=on';
2. Query OK,0 rows affected (0.00 sec)
5. 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 where1=1and 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;

立刻就返回数据了。

小结

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，只不过是正好将遇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。

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，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是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BUG。这里和大家分享下防止大家也遇到同样的坑。

案例分析

环境描述

数据库版本MySQL5.6.35

SQL语句

1. 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 where1=1and 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;

分析过程

当时凌晨4点左右客户打来电话告知数据库查询不到数据了非常着急，我们赶到了现场，当时的现象是这样的:

1. 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 where1=1and 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，我当时不相信就执行了一下:

1. 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 where1=1and 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. Emptyset(0.41 sec)
5. 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 where1=1and 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;
6. +----------+
7. | count(*)|
8. +----------+
9. |475|
10. +----------+
11. 1 row inset(0.41 sec)

当时也是比较慌张了，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

1. 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 where1=1and 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. | id | select_type | table | type | possible_keys | key | key_len |ref| rows |Extra|
4. +----+-------------+------------+--------+------------------------+-------------------+---------+-------------------+-------+-----------------------------+
5. |1| PRIMARY |<derived2>|ref|<auto_key0>|<auto_key0>|153|const|10|Usingwhere;Using filesort |
6. |2| DERIVED | o | range | idx_orderdatetime | idx_orderdatetime |6| NULL |46104|Using index condition |
7. |2| DERIVED | mm | eq_ref | PRIMARY,idx_memberid | PRIMARY |8| mall.o.buyerid |1| NULL |
8. |2| DERIVED | ms |ref| idx_userid | idx_userid |9| mall.o.salerid |1| NULL |
9. |2| DERIVED | mmt | eq_ref | PRIMARY,idx_merchantid | PRIMARY |8| mall.o.salerid |1| NULL |
10. |2| DERIVED | ma | eq_ref | PRIMARY | PRIMARY |8| mall.o.activityid |1| NULL |
11. |2| DERIVED | md |ref| idx_activityid | idx_activityid |8| mall.ma.actid |1| NULL |
12. +----+-------------+------------+--------+------------------------+-------------------+---------+-------------------+-------+-----------------------------+
13. 7 rows inset(0.00 sec)

新系统

1. 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 where1=1and 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. | id | select_type | table | type | possible_keys | key | key_len |ref| rows |Extra|
4. +----+-------------+------------+--------+------------------------+-------------------+---------+-------------------+-------+----------------------------------+
5. |1| PRIMARY |<derived2>|ref|<auto_key0>|<auto_key0>|153|const|10|Usingwhere;Using filesort |
6. |2| DERIVED | o | range | idx_orderdatetime | idx_orderdatetime |6| NULL |46104|Using index condition;Using MRR |
7. |2| DERIVED | mm | eq_ref | PRIMARY,idx_memberid | PRIMARY |8| mall.o.buyerid |1| NULL |
8. |2| DERIVED | ms |ref| idx_userid | idx_userid |9| mall.o.salerid |1| NULL |
9. |2| DERIVED | mmt | eq_ref | PRIMARY,idx_merchantid | PRIMARY |8| mall.o.salerid |1| NULL |
10. |2| DERIVED | ma | eq_ref | PRIMARY | PRIMARY |8| mall.o.activityid |1| NULL |
11. |2| DERIVED | md |ref| idx_activityid | idx_activityid |8| mall.ma.actid |1| NULL |
12. +----+-------------+------------+--------+------------------------+-------------------+---------+-------------------+-------+----------------------------------+
13. 7 rows inset(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，是不是有人修改了他，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:

1. set optimizer_switch='mrr_cost_based=on';
2. Query OK,0 rows affected (0.00 sec)
5. 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 where1=1and 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;.

立刻就返回数据了。

小结

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，只不过是正好将遇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。