MySQL Innodb如何找出阻塞事务源头SQL
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在mysql数据库中出现了阻塞问题，如何快速查找定位问题根源？在实验开始前，我们先梳理一下有什么工具或命令查看MySQL的阻塞，另外，我们也要一一对比其优劣，因为有些命令可能在实际环境下可能并不适用。
1： show engine innodb status
2： Innotop工具
3： INNODB_TRX 等系统表
下面我们理论联系实际，通过实验来测试总结一下这个问题。首先构造测试环境，数据库测试环境为（ 5.7.21 MySQL Community Server 和5.6.20-enterprise-commercial，这两个测试环境我都测试验证过）
mysql> use MyDB;Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A
Database changedmysql> create table test_blocking(id int primary key, name varchar(12));
Query OK, 0 rows affected (0.05 sec)mysql> insert into test_blocking-> select 1, \'kerry\' from dual;
Query OK, 1 row affected (0.00 sec)Records: 1 Duplicates: 0 Warnings: 0
mysql> insert into test_blocking-> select 2, \'jimmy\' from dual;
Query OK, 1 row affected (0.00 sec)Records: 1 Duplicates: 0 Warnings: 0
mysql> insert into test_blocking-> select 3, \'kkk\' from dual;
Query OK, 1 row affected (0.00 sec)Records: 1 Duplicates: 0 Warnings: 0
准备好测试环境数据后，那么我们接下来开始实验，为了实验效果，我们先将参数innodb_lock_wait_timeout设置为100。
mysql> show variables like \'innodb_lock_wait_timeout\';
+--------------------------+-------+
| Variable_name | Value |+--------------------------+-------+
| innodb_lock_wait_timeout | 50 |
+--------------------------+-------+
1 row in set (0.00 sec)
mysql> set global innodb_lock_wait_timeout=100 ;
Query OK, 0 rows affected (0.00 sec)mysql> select connection_id() from dual;
+-----------------+
| connection_id() |
+-----------------+
| 8 |
+-----------------+
1 row in set (0.00 sec)
mysql> set session autocommit=0;
Query OK, 0 rows affected (0.00 sec)mysql> select * from test_blocking where id=1 for update;
+----+-------+
| id | name |
+----+-------+
| 1 | kerry |
+----+-------+
1 row in set (0.00 sec)
然后在第二个连接会话中执行更新脚本，构造被阻塞的案例
mysql> select connection_id() from dual;
+-----------------+
| connection_id() |
+-----------------+
| 9 |
+-----------------+
1 row in set (0.00 sec)
mysql> update test_blocking set name=\'kk\' where id=1;
在第三个连接会话执行下面命令，查看TRANSACTIONS相关信息：
mysql> show engine innodb status\\G;
使用show engine innodb status命令后，可以查看其输出的TRANSACTIONS部分信息，如上截图所示，找到类似TRX HAS BEEN WATING ...部分的信息，
通过那部分信息，我们可以看到update test_blocking set name=\'kk\' where id=1这个SQL语句被阻塞了14秒，一直在等待获取X Lock。
TRANSACTIONS
------------
Trx id counter 148281 #下一个事务ID
Purge done for trx\'s n:o < 148273 undo n:o < 0 state: running but idle
History list length 552
LIST OF TRANSACTIONS FOR EACH SESSION:
---TRANSACTION 0, not started
MySQL thread id 15, OS thread handle 0x4cc64940, query id 261 localhost root cleaning up
---TRANSACTION 0, not started
MySQL thread id 14, OS thread handle 0x4cbe2940, query id 278 localhost root init
show engine innodb status
---TRANSACTION 148280, ACTIVE 24 sec
2 lock struct(s), heap size 360, 1 row lock(s)
MySQL thread id 8, OS thread handle 0x4cba1940, query id 276 localhost root cleaning up
---TRANSACTION 148279, ACTIVE 313 sec starting index read
mysql tables in use 1, locked 1
LOCK WAIT 2 lock struct(s), heap size 360, 1 row lock(s)
MySQL thread id 9, OS thread handle 0x4cc23940, query id 277 localhost root updating #线程ID为9， 操作系统线程句柄为0x4cc23940， 查询ID为277，账号为root的UPDATE操作
update test_blocking set name=\'kk\' where id=1 #具体SQL语句
------- TRX HAS BEEN WAITING 14 SEC FOR THIS LOCK TO BE GRANTED: #TRX等待授予锁已经有14秒了
RECORD LOCKS space id 337 page no 3 n bits 72 index `PRIMARY` of table `MyDB`.`test_blocking` trx id 148279 lock_mode X locks rec but not gap waiting
#在space id=337（test_blocking表的表空间），page no=3的页上，表test_blocking上的主键索引在等待X锁
Record lock, heap no 2 PHYSICAL RECORD: n_fields 4; compact format; info bits 0
0: len 4; hex 80000001; asc ;; #第一个字段是主键，制度按长为4，值为1
1: len 6; hex 000000024322; asc C";; #该字段为6个字节的事务id，这个id表示最近一次被更新的事务id（对应十进制为148258）
2: len 7; hex 9a000001f20110; asc ;; #该字段为7个字节的回滚指针，用于mvcc
3: len 5; hex 6b65727279; asc kerry;; #该字段表示的是此记录的第二个字段，长度为5，值为kerry（如果表有多个字段，那么此处后面还有记录）
mysql> select * from information_schema.INNODB_SYS_DATAFILES where space=337;
+-------+--------------------------+
| SPACE | PATH |
+-------+--------------------------+
| 337 | ./MyDB/test_blocking.ibd |
+-------+--------------------------+
1 row in set (0.00 sec)
mysql>
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