元数据锁（Metadata Lock，以下简称MDL）计划写三篇，这篇主要是介绍MDL的引入背景和基本概念，后两篇会着重介绍MySQL 5.7（包括8.0）及5.6中如何定位MDL问题。
在线上进行DDL操作时，相对于其可能带来的系统负载，其实，我们最担心的还是MDL其可能导致的阻塞问题。
一旦DDL操作因获取不到MDL被阻塞，后续其它针对该表的其它操作都会被阻塞。典型如下，如阻塞稍久的话，我们会看到Threads_running飙升，CPU告警。
mysql> show processlist;
+----+-----------------+-----------+-----------+---------+------+---------------------------------+------------------------------------+
| Id | User | Host | db | Command | Time | State | Info |
+----+-----------------+-----------+-----------+---------+------+---------------------------------+------------------------------------+
| 4 | event_scheduler | localhost | NULL | Daemon | 122 | Waiting on empty queue | NULL |
| 9 | root | localhost | NULL | Sleep | 57 | | NULL |
| 12 | root | localhost | employees | Query | 40 | Waiting for table metadata lock | alter table slowtech.t1 add c1 int |
| 13 | root | localhost | employees | Query | 35 | Waiting for table metadata lock | select * from slowtech.t1 |
| 14 | root | localhost | employees | Query | 30 | Waiting for table metadata lock | select * from slowtech.t1 |
| 15 | root | localhost | employees | Query | 19 | Waiting for table metadata lock | select * from slowtech.t1 |
| 16 | root | localhost | employees | Query | 10 | Waiting for table metadata lock | select * from slowtech.t1 |
| 17 | root | localhost | employees | Query | 0 | starting | show processlist |
+----+-----------------+-----------+-----------+---------+------+---------------------------------+------------------------------------+
rows in set (0.00 sec)
如果发生在线上，无疑会影响到业务。所以，一般建议将DDL操作放到业务低峰期做，其实有两方面的考虑，1. 避免对系统负载产生较大影响。2. 减少DDL被阻塞的概率。
MDL引入的背景
MDL是MySQL 5.5.3引入的，主要用于解决两个问题，
RR事务隔离级别下不可重复读的问题
如下所示，演示环境，MySQL 5.5.0。
session1> begin;
Query OK, 0 rows affected (0.00 sec)
session1> select * from t1;
+------+------+
| id | name |
+------+------+
| 1 | a |
| 2 | b |
+------+------+
rows in set (0.00 sec)
session2> alter table t1 add c1 int;
Query OK, 2 rows affected (0.02 sec)
Records: 2 Duplicates: 0 Warnings: 0
session1> select * from t1;
Empty set (0.00 sec)
session1> commit;
Query OK, 0 rows affected (0.00 sec)
session1> select * from t1;
+------+------+------+
| id | name | c1 |
+------+------+------+
| 1 | a | NULL |
| 2 | b | NULL |
+------+------+------+
rows in set (0.00 sec)
可以看到，虽然是RR隔离级别，但在开启事务的情况下，第二次查询却没有结果。
主从复制问题
包括主从数据不一致，主从复制中断等。 如下面的主从数据不一致。
session1> create table t1(id int,name varchar(10)) engine=innodb;
Query OK, 0 rows affected (0.00 sec)
session1> begin;
Query OK, 0 rows affected (0.00 sec)
session1> insert into t1 values(1,'a');
Query OK, 1 row affected (0.00 sec)
session2> truncate table t1;
Query OK, 0 rows affected (0.46 sec)
session1> commit;
Query OK, 0 rows affected (0.35 sec)
session1> select * from t1;
Empty set (0.00 sec)
再来看看从库的结果
session1> select * from slowtech.t1;
+------+------+------+
| id | name | c1 |
+------+------+------+
| 1 | a | NULL |
+------+------+------+
row in set (0.00 sec)
看看binlog的内容，可以看到，truncate操作记录在前，insert操作记录在后。
# at 7140
#180714 19:32:14 server id 1 end_log_pos 7261 Query thread_id=31 exec_time=0 error_code=0
SET TIMESTAMP=1531567934/*!*/;
create table t1(id int,name varchar(10)) engine=innodb
/*!*/;
# at 7261
#180714 19:32:30 server id 1 end_log_pos 7333 Query thread_id=32 exec_time=0 error_code=0
SET TIMESTAMP=1531567950/*!*/;
BEGIN
/*!*/;
# at 7333
#180714 19:32:30 server id 1 end_log_pos 7417 Query thread_id=32 exec_time=0 error_code=0
SET TIMESTAMP=1531567950/*!*/;
truncate table t1
/*!*/;
# at 7417
#180714 19:32:30 server id 1 end_log_pos 7444 Xid = 422
COMMIT/*!*/;
# at 7444
#180714 19:32:34 server id 1 end_log_pos 7516 Query thread_id=31 exec_time=0 error_code=0
SET TIMESTAMP=1531567954/*!*/;
BEGIN
/*!*/;
# at 7516
#180714 19:32:24 server id 1 end_log_pos 7611 Query thread_id=31 exec_time=0 error_code=0
SET TIMESTAMP=1531567944/*!*/;
insert into t1 values(1,'a')
/*!*/;
# at 7611
#180714 19:32:34 server id 1 end_log_pos 7638 Xid = 421
COMMIT/*!*/;
如果会话2执行的是drop table操作，还会导致主从中断。
有意思的是，如果会话2执行的是alter table操作，其依旧会被阻塞，阻塞时间受innodb_lock_wait_timeout参数限制。
mysql> show processlist;
+----+------+-----------+----------+---------+------+-------------------+---------------------------+
| Id | User | Host | db | Command | Time | State | Info |
+----+------+-----------+----------+---------+------+-------------------+---------------------------+
| 54 | root | localhost | NULL | Query | 0 | NULL | show processlist |
| 58 | root | localhost | slowtech | Sleep | 1062 | | NULL |
| 60 | root | localhost | slowtech | Query | 11 | copy to tmp table | alter table t1 add c1 int |
+----+------+-----------+----------+---------+------+-------------------+---------------------------+
rows in set (0.00 sec)
MDL的基本概念
首先，看看官方的说法，
To ensure transaction serializability, the server must not permit one session to perform a data definition language (DDL) statement on a table that is used in an uncompleted explicitly or implicitly started transaction in another session.
The server achieves this by acquiring metadata locks on tables used within a transaction and deferring release of those locks until the transaction ends.
A metadata lock on a table prevents changes to the table's structure.
This locking approach has the implication that a table that is being used by a transaction within one session cannot be used in DDL statements by other sessions until the transaction ends.
从上面的描述可以看到，
1. MDL出现的初衷就是为了保护一个处于事务中的表的结构不被修改。
2. 这里提到的事务包括两类，显式事务和AC-NL-RO（auto-commit non-locking read-only）事务。显式事务包括两类：1. 关闭AutoCommit下的操作，2. 以begin或start transaction开始的操作。AC-NL-RO可理解为AutoCommit开启下的select操作。
3. MDL是事务级别的，只有在事务结束后才会释放。在此之前，其实也有类似的保护机制，只不过是语句级别的。
需要注意的是，MDL不仅仅适用于表，同样也适用于其它对象，如下表所示，其中，"等待状态"对应的是"show processlist"中的State。
为了提高数据库的并发度，MDL被细分为了11种类型。
-
MDL_INTENTION_EXCLUSIVE
-
MDL_SHARED
-
MDL_SHARED_HIGH_PRIO
-
MDL_SHARED_READ
-
MDL_SHARED_WRITE
-
MDL_SHARED_WRITE_LOW_PRIO
-
MDL_SHARED_UPGRADABLE
-
MDL_SHARED_READ_ONLY
-
MDL_SHARED_NO_WRITE
-
MDL_SHARED_NO_READ_WRITE
-
MDL_EXCLUSIVE
常用的有MDL_SHARED_READ,MDL_SHARE D_WRITE及MDL_EXCLUSIVE，其分别用于SELECT操作，DML操作及DDL操作。 其它类型的对应操作可参考源码sql/mdl.h。
对于MDL_EXCLUSIVE，官方的解释是，
An exclusive metadata lock.
A connection holding this lock can modify both table's metadata and data.
No other type of metadata lock can be granted while this lock is held.
To be used for CREATE/DROP/RENAME TABLE statements and for execution of certain phases of other DDL statements.
简而言之，MDL_EXCLUSIVE是独占锁，在其持有期间是不允许其它类型的MDL被授予，自然也包括SELECT和DML操作。
这也就是为什么DDL操作被阻塞时，后续其它操作也会被阻塞。
关于MDL的补充
1. MDL的最大等待时间由lock_wait_timeout参数决定，其默认值为31536000（365天）。在使用工具进行DDL操作时，这个值就不太合理。事实上，pt-online-schema-change和gh-ost对其就进行了相应的调整，其中，前者60s，后者3s。
2. 如果一个SQL语法上有效，但执行时报错，如，列名不存在，其同样会获取MDL锁，直到事务结束才释放。
MySQL 5.7中如何定位DDL被阻塞的问题
在上篇文章《MySQL表结构变更，不可不知的Metadata Lock》中，我们介绍了MDL引入的背景，及基本概念，从“道”的层面知道了什么是MDL。下面就从“术”的层面看看如何定位MDL的相关问题。
在MySQL 5.7中，针对MDL，引入了一张新表performance_schema.metadata_locks，该表可对外展示MDL的相关信息，包括其作用对象，类型及持有等待情况。
开启MDL的instrument
但是相关instrument并没有开启（MySQL 8.0是默认开启的），其可通过如下两种方式开启，
临时生效
修改performance_schema.setup_instrume nts表，但实例重启后，又会恢复为默认值。
UPDATE performance_schema.setup_instruments SET ENABLED = 'YES', TIMED = 'YES'
WHERE NAME = 'wait/lock/metadata/sql/mdl';
[mysqld]performance-schema-instrument='wait/lock/metadata/sql/mdl=ON'
测试场景
下面结合一个简单的Demo，来看看在MySQL 5.7中如何定位DDL操作的阻塞问题。
session1> begin;
Query OK, 0 rows affected (0.00 sec)
session1> delete from slowtech.t1 where id=2;
Query OK, 1 row affected (0.00 sec)
session1> select * from slowtech.t1;
+------+------+
| id | name |
+------+------+
| 1 | a |
+------+------+
row in set (0.00 sec)
session1> update slowtech.t1 set name='c' where id=1;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
session2> alter table slowtech.t1 add c1 int; ##被阻塞
session3> show processlist;
+----+------+-----------+------+---------+------+---------------------------------+------------------------------------+
| Id | User | Host | db | Command | Time | State | Info |
+----+------+-----------+------+---------+------+---------------------------------+------------------------------------+
| 2 | root | localhost | NULL | Sleep | 51 | | NULL |
| 3 | root | localhost | NULL | Query | 0 | starting | show processlist |
| 4 | root | localhost | NULL | Query | 9 | Waiting for table metadata lock | alter table slowtech.t1 add c1 int |
+----+------+-----------+------+---------+------+---------------------------------+------------------------------------+
rows in set (0.00 sec)
session3> select object_type,object_schema,object_name,lock_type,lock_duration,lock_status,owner_thread_id from performance_schema.metadata_locks;
+-------------+--------------------+----------------+---------------------+---------------+-------------+-----------------+
| object_type | object_schema | object_name | lock_type | lock_duration | lock_status | owner_thread_id |
+-------------+--------------------+----------------+---------------------+---------------+-------------+-----------------+
| TABLE | slowtech | t1 | SHARED_WRITE | TRANSACTION | GRANTED | 27 |
| GLOBAL | NULL | NULL | INTENTION_EXCLUSIVE | STATEMENT | GRANTED | 29 |
| SCHEMA | slowtech | NULL | INTENTION_EXCLUSIVE | TRANSACTION | GRANTED | 29 |
| TABLE | slowtech | t1 | SHARED_UPGRADABLE | TRANSACTION | GRANTED | 29 |
| TABLE | slowtech | t1 | EXCLUSIVE | TRANSACTION | PENDING | 29 |
| TABLE | performance_schema | metadata_locks | SHARED_READ | TRANSACTION | GRANTED | 28 |
+-------------+--------------------+----------------+---------------------+---------------+-------------+-----------------+
rows in set (0.00 sec)
这里，重点关注lock_status，"PENDING"代表线程在等待MDL，而"GRANTED"则代表线程持有MDL。
如何找出引起阻塞的会话
结合owner_thread_id，可以可到，是29号线程在等待27号线程的MDL，此时，可kill掉52号线程。
但需要注意的是，owner_thread_id给出的只是线程ID，并不是show processlist中的ID。如果要查找线程对应的processlist id，需查询performance_schema.threads表。
session3> select * from performance_schema.threads where thread_id in (27,29)\G
*************************** 1. row ***************************
THREAD_ID: 27
NAME: thread/sql/one_connection
TYPE: FOREGROUND
PROCESSLIST_ID: 2
PROCESSLIST_USER: root
PROCESSLIST_HOST: localhost
PROCESSLIST_DB: NULL
PROCESSLIST_COMMAND: Sleep
PROCESSLIST_TIME: 214
PROCESSLIST_STATE: NULL
PROCESSLIST_INFO: NULL
PARENT_THREAD_ID: 1
ROLE: NULL
INSTRUMENTED: YES
HISTORY: YES
CONNECTION_TYPE: Socket
THREAD_OS_ID: 9800
*************************** 2. row ***************************
THREAD_ID: 29
NAME: thread/sql/one_connection
TYPE: FOREGROUND
PROCESSLIST_ID: 4
PROCESSLIST_USER: root
PROCESSLIST_HOST: localhost
PROCESSLIST_DB: NULL
PROCESSLIST_COMMAND: Query
PROCESSLIST_TIME: 172
PROCESSLIST_STATE: Waiting for table metadata lock
PROCESSLIST_INFO: alter table slowtech.t1 add c1 int
PARENT_THREAD_ID: 1
ROLE: NULL
INSTRUMENTED: YES
HISTORY: YES
CONNECTION_TYPE: Socket
THREAD_OS_ID: 9907
rows in set (0.00 sec)
将这两张表结合，借鉴sys.innodb_lock _waits的输出，实际上我们也可以直观地呈现MDL的等待关系。
SELECT
a.OBJECT_SCHEMA AS locked_schema,
a.OBJECT_NAME AS locked_table,
"Metadata Lock" AS locked_type,
c.PROCESSLIST_ID AS waiting_processlist_id,
c.PROCESSLIST_TIME AS waiting_age,
c.PROCESSLIST_INFO AS waiting_query,
c.PROCESSLIST_STATE AS waiting_state,
d.PROCESSLIST_ID AS blocking_processlist_id,
d.PROCESSLIST_TIME AS blocking_age,
d.PROCESSLIST_INFO AS blocking_query,
concat('KILL ', d.PROCESSLIST_ID) AS sql_kill_blocking_connection
FROM
performance_schema.metadata_locks a
JOIN performance_schema.metadata_locks b ON a.OBJECT_SCHEMA = b.OBJECT_SCHEMA
AND a.OBJECT_NAME = b.OBJECT_NAME
AND a.lock_status = 'PENDING'
AND b.lock_status = 'GRANTED'
AND a.OWNER_THREAD_ID <> b.OWNER_THREAD_ID
AND a.lock_type = 'EXCLUSIVE'
JOIN performance_schema.threads c ON a.OWNER_THREAD_ID = c.THREAD_ID
JOIN performance_schema.threads d ON b.OWNER_THREAD_ID = d.THREAD_ID\G
*************************** 1. row ***************************
locked_schema: slowtech
locked_table: t1
locked_type: Metadata Lock
waiting_processlist_id: 4
waiting_age: 259
waiting_query: alter table slowtech.t1 add c1 int
waiting_state: Waiting for table metadata lock
blocking_processlist_id: 2
blocking_age: 301
blocking_query: NULL
sql_kill_blocking_connection: KILL 2
row in set (0.00 sec)
输出一目了然，DDL操作如果要获得MDL，执行kill 2即可。
官方的sys.schematablelock_waits
实际上，MySQL 5.7在sys库中也集成了类似功能，同样的场景，其输出如下，
mysql> select * from sys.schema_table_lock_waits\G
*************************** 1. row ***************************
object_schema: slowtech
object_name: t1
waiting_thread_id: 29
waiting_pid: 4
waiting_account: root@localhost
waiting_lock_type: EXCLUSIVE
waiting_lock_duration: TRANSACTION
waiting_query: alter table slowtech.t1 add c1 int
waiting_query_secs: 446
waiting_query_rows_affected: 0
waiting_query_rows_examined: 0
blocking_thread_id: 27
blocking_pid: 2
blocking_account: root@localhost
blocking_lock_type: SHARED_READ
blocking_lock_duration: TRANSACTION
sql_kill_blocking_query: KILL QUERY 2
sql_kill_blocking_connection: KILL 2
*************************** 2. row ***************************
object_schema: slowtech
object_name: t1
waiting_thread_id: 29
waiting_pid: 4
waiting_account: root@localhost
waiting_lock_type: EXCLUSIVE
waiting_lock_duration: TRANSACTION
waiting_query: alter table slowtech.t1 add c1 int
waiting_query_secs: 446
waiting_query_rows_affected: 0
waiting_query_rows_examined: 0
blocking_thread_id: 29
blocking_pid: 4
blocking_account: root@localhost
blocking_lock_type: SHARED_UPGRADABLE
blocking_lock_duration: TRANSACTION
sql_kill_blocking_query: KILL QUERY 4
sql_kill_blocking_connection: KILL 4
rows in set (0.00 sec)
具体分析下官方的输出，
只有一个alter table操作，却产生了两条记录，而且两条记录的kill对象竟然还不一样，对表结构不熟悉及不仔细看记录内容的话，难免会kill错对象。
不仅如此，如果有N个查询被DDL操作堵塞，则会产生N*2条记录。在阻塞操作较多的情况下，这N*2条记录完全是个噪音。
而之前的SQL，无论有多少操作被阻塞，一个alter table操作，就只会输出一条记录。
如何查看阻塞会话已经执行过的操作
但上面这个SQL也有遗憾，其blocking_query为NULL，而在会话1中，其明明已经执行了三个SQL。
这个与performance_schema.threads（类似于show processlist）有关，其只会输出当前正在运行的SQL，对于已经执行过的，实际上是没办法看到。
但在线上，kill是一个需要谨慎的操作，毕竟你很难知道kill的是不是业务关键操作？又或者，是个批量update操作？那么，有没有办法抓到该事务之前的操作呢？
答案，有。
即Performance Schema中记录Statement Event（操作事件）的表，具体包括events_statements_current,events_statements_history,events_statements_history_long,prepared_statements_instances。
常用的是前面三个。
三者的表结构完全一致，其中，events_statements_history又包含了events_statements_current的操作，所以我们这里会使用events_statements_history。
终极SQL如下，
SELECT
locked_schema,
locked_table,
locked_type,
waiting_processlist_id,
waiting_age,
waiting_query,
waiting_state,
blocking_processlist_id,
blocking_age,
substring_index(sql_text,"transaction_begin;" ,-1) AS blocking_query,
sql_kill_blocking_connection
FROM
(
SELECT
b.OWNER_THREAD_ID AS granted_thread_id,
a.OBJECT_SCHEMA AS locked_schema,
a.OBJECT_NAME AS locked_table,
"Metadata Lock" AS locked_type,
c.PROCESSLIST_ID AS waiting_processlist_id,
c.PROCESSLIST_TIME AS waiting_age,
c.PROCESSLIST_INFO AS waiting_query,
c.PROCESSLIST_STATE AS waiting_state,
d.PROCESSLIST_ID AS blocking_processlist_id,
d.PROCESSLIST_TIME AS blocking_age,
d.PROCESSLIST_INFO AS blocking_query,
concat('KILL ', d.PROCESSLIST_ID) AS sql_kill_blocking_connection
FROM
performance_schema.metadata_locks a
JOIN performance_schema.metadata_locks b ON a.OBJECT_SCHEMA = b.OBJECT_SCHEMA
AND a.OBJECT_NAME = b.OBJECT_NAME
AND a.lock_status = 'PENDING'
AND b.lock_status = 'GRANTED'
AND a.OWNER_THREAD_ID <> b.OWNER_THREAD_ID
AND a.lock_type = 'EXCLUSIVE'
JOIN performance_schema.threads c ON a.OWNER_THREAD_ID = c.THREAD_ID
JOIN performance_schema.threads d ON b.OWNER_THREAD_ID = d.THREAD_ID
) t1,
(
SELECT
thread_id,
group_concat( CASE WHEN EVENT_NAME = 'statement/sql/begin' THEN "transaction_begin" ELSE sql_text END ORDER BY event_id SEPARATOR ";" ) AS sql_text
FROM
performance_schema.events_statements_history
GROUP BY thread_id
) t2
WHERE
t1.granted_thread_id = t2.thread_id \G
*************************** 1. row ***************************
locked_schema: slowtech
locked_table: t1
locked_type: Metadata Lock
waiting_processlist_id: 4
waiting_age: 294
waiting_query: alter table slowtech.t1 add c1 int
waiting_state: Waiting for table metadata lock
blocking_processlist_id: 2
blocking_age: 336
blocking_query: delete from slowtech.t1 where id=2;select * from slowtech.t1;update slowtech.t1 set name='c' where id=1
sql_kill_blocking_connection: KILL 2
row in set, 1 warning (0.00 sec)
从上面的输出可以看到，blocking_query中包含了会话1中当前事务的所有操作，按执行的先后顺序输出。
需要注意的是，默认情况下，events_statements_history只会保留每个线程最近的10个操作，如果事务中进行的操作较多，实际上也是没办法抓全的。
Anyway, it is better than nothing！
MySQL 5.6中如何定位DDL被阻塞的问题
在上一篇文章《MySQL 5.7中如何定位DDL被阻塞的问题》中，对于DDL被阻塞问题的定位，我们主要是基于MySQL 5.7新引入的performance_schema.metadata_locks表。提出的定位方法，颇有种"锦上添花"的意味，而且，也只适用于MySQL 5.7开始的版本。
但在实际生产中，MySQL 5.6还是占绝不多数。虽然MySQL 8.0都已经GA了，但鉴于数据库的特殊性，在对待升级的这个事情上，相当一部分人还是秉持着一种“不主动”的态度。
既然MySQL 5.6用者众多，有没有一种方法，来解决MySQL 5.6的这个痛点呢？
还是之前的测试Demo
会话1开启了事务并执行了三个操作，但未提交，此时，会话2执行了alter table操作，被阻塞。
session1> begin;
Query OK, 0 rows affected (0.00 sec)
session1> delete from slowtech.t1 where id=2;
Query OK, 1 row affected (0.00 sec)
session1> select * from slowtech.t1;
+------+------+
| id | name |
+------+------+
| 1 | a |
+------+------+
row in set (0.00 sec)
session1> update slowtech.t1 set name='c' where id=1;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
session2> alter table slowtech.t1 add c1 int; ##被阻塞
session3> show processlist;
+----+------+-----------+------+---------+------+---------------------------------+------------------------------------+
| Id | User | Host | db | Command | Time | State | Info |
+----+------+-----------+------+---------+------+---------------------------------+------------------------------------+
| 2 | root | localhost | NULL | Sleep | 51 | | NULL |
| 3 | root | localhost | NULL | Query | 0 | starting | show processlist |
| 4 | root | localhost | NULL | Query | 9 | Waiting for table metadata lock | alter table slowtech.t1 add c1 int |
+----+------+-----------+------+---------+------+---------------------------------+------------------------------------+
rows in set (0.00 sec)
其实，导致DDL阻塞的操作，无非两类：
1. 慢查询
2. 表上有事务未提交
其中，第一类比较好定位，通过show processlist即能发现。而第二类基本没法定位，因为未提交事务的连接在show processlist中的输出同空闲连接一样。
如下面Id为2的连接，虽然Command显示为“Sleep”，其实是事务未提交。
mysql> show processlist;
+----+------+-----------+------+---------+------+---------------------------------+------------------------------------+
| Id | User | Host | db | Command | Time | State | Info |
+----+------+-----------+------+---------+------+---------------------------------+------------------------------------+
| 2 | root | localhost | NULL | Sleep | 77 | | NULL |
| 3 | root | localhost | NULL | Query | 0 | starting | show processlist |
| 4 | root | localhost | NULL | Query | 44 | Waiting for table metadata lock | alter table slowtech.t1 add c1 int |
+----+------+-----------+------+---------+------+---------------------------------+------------------------------------+
rows in set (0.00 sec)
所以，网上有kill空闲（Command为Sleep）连接的说法，其实也不无道理，但这样做就太简单粗暴了，难免会误杀。
其实，既然是事务，在information_schema. innodb_trx中肯定会有记录，如会话1中的事务，在表中的记录如下，
mysql> select * from information_schema.innodb_trx\G
*************************** 1. row ***************************
trx_id: 1050390
trx_state: RUNNING
trx_started: 2018-07-17 08:55:32
trx_requested_lock_id: NULL
trx_wait_started: NULL
trx_weight: 4
trx_mysql_thread_id: 2
trx_query: NULL
trx_operation_state: NULL
trx_tables_in_use: 0
trx_tables_locked: 1
trx_lock_structs: 2
trx_lock_memory_bytes: 1136
trx_rows_locked: 3
trx_rows_modified: 2
trx_concurrency_tickets: 0
trx_isolation_level: REPEATABLE READ
trx_unique_checks: 1
trx_foreign_key_checks: 1
trx_last_foreign_key_error: NULL
trx_adaptive_hash_latched: 0
trx_adaptive_hash_timeout: 0
trx_is_read_only: 0
trx_autocommit_non_locking: 0
row in set (0.00 sec)
其中trx_mysql_thread_id是线程id，结合performance_schema.threads，可以知道当前哪些连接上存在着活跃事务，这样就进一步缩小了可被kill的线程范围。
但从影响程度上，和kill所有Command为Sleep的连接没太大区别，毕竟，kill真正的空闲连接对业务的影响不大。
此时，依然可以借助performance_schema. events_statements_history表。
在上篇MySQL 5.7的分析中，我们是首先知道引发阻塞的线程ID，然后利用events_statements_history表，查看该线程的相关SQL。
而在MySQL 5.6中，我们并不知道引发阻塞的线程ID，但是，我们可以反其道而行之，利用穷举法，首先统计出所有线程在当前事务执行过的所有SQL，然后再判断这些SQL中是否包含目标表。
具体SQL如下，
SELECT
processlist_id,
sql_text
FROM
(
SELECT
c.processlist_id,
substring_index( sql_text, "transaction_begin;",-1 ) sql_text
FROM
information_schema.innodb_trx a,
(
SELECT
thread_id,
group_concat( CASE WHEN EVENT_NAME = 'statement/sql/begin' THEN "transaction_begin" ELSE sql_text END ORDER BY event_id SEPARATOR ";" ) AS sql_text
FROM
performance_schema.events_statements_history
GROUP BY
thread_id
) b,
performance_schema.threads c
WHERE
a.trx_mysql_thread_id = c.processlist_id
AND b.thread_id = c.thread_id
) t
WHERE
sql_text LIKE '%t1%';
+----------------+---------------------------------------------------------------------------------------------------------+
| processlist_id | sql_text |
+----------------+---------------------------------------------------------------------------------------------------------+
| 2 | delete from slowtech.t1 where id=2;select * from slowtech.t1;update slowtech.t1 set name='c' where id=1 |
+----------------+---------------------------------------------------------------------------------------------------------+
row in set (0.01 sec)
从输出来看，确实也达到了预期效果。
需要注意的是，在MySQL5.6中，events_statements_history默认是没有开启的。
mysql> SELECT * FROM performance_schema.setup_consumers WHERE NAME LIKE '%statements%';
+--------------------------------+---------+
| NAME | ENABLED |
+--------------------------------+---------+
| events_statements_current | YES |
| events_statements_history | NO |
| events_statements_history_long | NO |
| statements_digest | YES |
+--------------------------------+---------+
rows in set (0.00 sec)
