optimizer_switch引起的诡异问题

﹏ヽ暗。殇╰゛Y 2022-06-14 04:18 217阅读 0赞

### 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 ####

#### 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:

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:

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)

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

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 | <derived2> | ref | <auto_key0> | <auto_key0> | 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 | <derived2> | ref | <auto_key0> | <auto_key0> | 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:

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 ###

### 案例分析 ###

#### 环境描述 ####

数据库版本MySQL5.6.35

#### SQL语句 ####

#### 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:

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:

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 | <derived2> | ref | <auto_key0> | <auto_key0> | 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)

立刻就返回数据了。

### 小结 ###