前言：
本篇文章主要介绍mysql长事务相关内容，比如说我们开启的一个事务，一直没提交或回滚会怎样呢，出现事务等待情况应该如何处理，本篇文章将给你答案。
注意：本篇文章并不聚焦于谈论事务隔离级别以及相关特性。而是介绍长事务相关危害以及监控处理方法。本文是基于mysql5.7.23版本，不可重复读(rr)隔离级别所做实验。(语句为\g可以使查询结构显示更易读，但只可以在mysql命令行使用。)
1.什么是长事务
首先我们先要知道什么是长事务，顾名思义就是运行时间比较长，长时间未提交的事务，也可以称之为大事务。这类事务往往会造成大量的阻塞和锁超时，容易造成主从延迟，要尽量避免使用长事务。
下面我将演示下如何开启事务及模拟长事务：
#假设我们有一张stu_tb表，结构及数据如下
mysql> show create table stu_tb\g
*************************** 1. row ***************************
table: stu_tb
create table: create table `stu_tb` (
`increment_id` int(11) not null auto_increment comment '自增主键',
`stu_id` int(11) not null comment '学号',
`stu_name` varchar(20) default null comment '学生姓名',
`create_time` timestamp not null default current_timestamp comment '创建时间',
`update_time` timestamp not null default current_timestamp on update current_timestamp comment '修改时间',
primary key (`increment_id`),
unique key `uk_stu_id` (`stu_id`) using btree
) engine=innodb auto_increment=9 default charset=utf8 comment='测试学生表'
1 row in set (0.01 sec)
mysql> select * from stu_tb;
+--------------+--------+----------+---------------------+---------------------+
| increment_id | stu_id | stu_name | create_time | update_time |
+--------------+--------+----------+---------------------+---------------------+
| 1 | 1001 | from1 | 2019-09-15 14:27:34 | 2019-09-15 14:27:34 |
| 2 | 1002 | dfsfd | 2019-09-15 14:27:34 | 2019-09-15 14:27:34 |
| 3 | 1003 | fdgfg | 2019-09-15 14:27:34 | 2019-09-15 14:27:34 |
| 4 | 1004 | sdfsdf | 2019-09-15 14:27:34 | 2019-09-15 14:27:34 |
| 5 | 1005 | dsfsdg | 2019-09-15 14:27:34 | 2019-09-15 14:27:34 |
| 6 | 1006 | fgd | 2019-09-15 14:27:34 | 2019-09-15 14:27:34 |
| 7 | 1007 | fgds | 2019-09-15 14:27:34 | 2019-09-15 14:27:34 |
| 8 | 1008 | dgfsa | 2019-09-15 14:27:34 | 2019-09-15 14:27:34 |
+--------------+--------+----------+---------------------+---------------------+
8 rows in set (0.00 sec)
#显式开启事务，可用begin或start transaction
mysql> start transaction;
query ok, 0 rows affected (0.00 sec)
mysql> select * from stu_tb where stu_id = 1006 for update;
+--------------+--------+----------+---------------------+---------------------+
| increment_id | stu_id | stu_name | create_time | update_time |
+--------------+--------+----------+---------------------+---------------------+
| 6 | 1006 | fgd | 2019-09-15 14:27:34 | 2019-09-15 14:27:34 |
+--------------+--------+----------+---------------------+---------------------+
1 row in set (0.01 sec)
#如果我们不及时提交上个事务，那么这个事务就变成了长事务，当其他会话要操作这条数据时，就会一直等待。
2.如何找到长事务
遇到事务等待问题时，我们首先要做的是找到正在执行的事务。information_schema.innodb_trx 表中包含了当前innodb内部正在运行的事务信息，这个表中给出了事务的开始时间，我们可以稍加运算即可得到事务的运行时间。
mysql> select t.*,to_seconds(now())-to_seconds(t.trx_started) idle_time from information_schema.innodb_trx t \g
*************************** 1. row ***************************
trx_id: 6168
trx_state: running
trx_started: 2019-09-16 11:08:27
trx_requested_lock_id: null
trx_wait_started: null
trx_weight: 3
trx_mysql_thread_id: 11
trx_query: null
trx_operation_state: null
trx_tables_in_use: 0
trx_tables_locked: 1
trx_lock_structs: 3
trx_lock_memory_bytes: 1136
trx_rows_locked: 2
trx_rows_modified: 0
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
idle_time: 170
在结果中idle_time是计算产生的，也是事务的持续时间。但事务的trx_query是nul，这并不是说事务什么也没执行，一个事务可能包含多个sql，如果sql执行完毕就不再显示了。当前事务正在执行，innodb也不知道这个事务后续还有没有sql,啥时候会commit。因此trx_query不能提供有意义的信息。
如果我们想看到这个事务执行过的sql，看是否可以杀掉长事务，怎么办呢？我们可以联合其他系统表查询得到，具体查询sql如下：
mysql> select now(),(unix_timestamp(now()) - unix_timestamp(a.trx_started)) diff_sec,b.id,b.user,b.host,b.db,d.sql_text from information_schema.innodb_trx a inner join -> information_schema.processlist b -> on a.trx_mysql_thread_id=b.id and b.command = 'sleep' -> inner join performance_schema.threads c on b.id = c.processlist_id -> inner join performance_schema.events_statements_current d on d.thread_id = c.thread_id; +---------------------+----------+----+------+-----------+--------+-----------------------------------------------------+ | now() | diff_sec | id | user | host | db | sql_text | +---------------------+----------+----+------+-----------+--------+-----------------------------------------------------+ | 2019-09-16 14:06:26 | 54 | 17 | root | localhost | testdb | select * from stu_tb where stu_id = 1006 for update | +---------------------+----------+----+------+-----------+--------+-----------------------------------------------------+
上述结果中diff_sec和上面idle_time表示意思相同，都是代表此事务持续的秒数。sql_text表示该事务刚执行的sql。但是呢，上述语句只能查到事务最后执行的sql，我们知道，一个事务里可能包含多个sql，那我们想查询这个未提交的事务执行过哪些sql，是否可以满足呢，答案是结合events_statements_history系统表也可以满足需求。下面语句将会查询出该事务执行过的所有sql：
mysql> select -> ps.id 'process id', -> ps.user, -> ps.host, -> esh.event_id, -> trx.trx_started, -> esh.event_name 'event name', -> esh.sql_text 'sql', -> ps.time -> from -> performance_schema.events_statements_history esh -> join performance_schema.threads th on esh.thread_id = th.thread_id -> join information_schema.processlist ps on ps.id = th.processlist_id -> left join information_schema.innodb_trx trx on trx.trx_mysql_thread_id = ps.id -> where -> trx.trx_id is not null -> and ps.user != 'system_user' -> order by -> esh.event_id; +------------+------+-----------+----------+---------------------+------------------------------+-----------------------------------------------------+------+ | process id | user | host | event_id | trx_started | event name | sql | time | +------------+------+-----------+----------+---------------------+------------------------------+-----------------------------------------------------+------+ | 20 | root | localhost | 1 | 2019-09-16 14:18:44 | statement/sql/select | select @@version_comment limit 1 | 60 | | 20 | root | localhost | 2 | 2019-09-16 14:18:44 | statement/sql/begin | start transaction | 60 | | 20 | root | localhost | 3 | 2019-09-16 14:18:44 | statement/sql/select | select database() | 60 | | 20 | root | localhost | 4 | 2019-09-16 14:18:44 | statement/com/init db | null | 60 | | 20 | root | localhost | 5 | 2019-09-16 14:18:44 | statement/sql/show_databases | show databases | 60 | | 20 | root | localhost | 6 | 2019-09-16 14:18:44 | statement/sql/show_tables | show tables | 60 | | 20 | root | localhost | 7 | 2019-09-16 14:18:44 | statement/com/field list | null | 60 | | 20 | root | localhost | 8 | 2019-09-16 14:18:44 | statement/com/field list | null | 60 | | 20 | root | localhost | 9 | 2019-09-16 14:18:44 | statement/sql/select | select * from stu_tb | 60 | | 20 | root | localhost | 10 | 2019-09-16 14:18:44 | statement/sql/select | select * from stu_tb where stu_id = 1006 for update | 60 | +------------+------+-----------+----------+---------------------+------------------------------+-----------------------------------------------------+------+
从上述结果中我们可以看到该事务从一开始到现在执行过的所有sql，当我们把该事务相关信息都查询清楚后，我们就可以判定该事务是否可以杀掉，以免影响其他事务造成等待现象。
在这里稍微拓展下，长事务极易造成阻塞或者死锁现象，通常情况下我们可以首先查询 sys.innodb_lock_waits 视图确定有没有事务阻塞现象：
#假设一个事务执行 select * from stu_tb where stu_id = 1006 for update
#另外一个事务执行 update stu_tb set stu_name = 'wang' where stu_id = 1006
mysql> select * from sys.innodb_lock_waits\g
*************************** 1. row ***************************
wait_started: 2019-09-16 14:34:32
wait_age: 00:00:03
wait_age_secs: 3
locked_table: `testdb`.`stu_tb`
locked_index: uk_stu_id
locked_type: record
waiting_trx_id: 6178
waiting_trx_started: 2019-09-16 14:34:32
waiting_trx_age: 00:00:03
waiting_trx_rows_locked: 1
waiting_trx_rows_modified: 0
waiting_pid: 19
waiting_query: update stu_tb set stu_name = 'wang' where stu_id = 1006
waiting_lock_id: 6178:47:4:7
waiting_lock_mode: x
blocking_trx_id: 6177
blocking_pid: 20
blocking_query: null
blocking_lock_id: 6177:47:4:7
blocking_lock_mode: x
blocking_trx_started: 2019-09-16 14:18:44
blocking_trx_age: 00:15:51
blocking_trx_rows_locked: 2
blocking_trx_rows_modified: 0
sql_kill_blocking_query: kill query 20
sql_kill_blocking_connection: kill 20
上述结果显示出被阻塞的sql以及锁的类型，更强大的是杀掉会话的语句也给出来了。但是并没有找到阻塞会话执行的sql，如果我们想找出更详细的信息，可以使用下面语句：
mysql> select -> tmp.*, -> c.sql_text blocking_sql_text, -> p.host blocking_host -> from -> ( -> select -> r.trx_state wating_trx_state, -> r.trx_id waiting_trx_id, -> r.trx_mysql_thread_id waiting_thread, -> r.trx_query waiting_query, -> b.trx_state blocking_trx_state, -> b.trx_id blocking_trx_id, -> b.trx_mysql_thread_id blocking_thread, -> b.trx_query blocking_query -> from -> information_schema.innodb_lock_waits w -> inner join information_schema.innodb_trx b on b.trx_id = w.blocking_trx_id -> inner join information_schema.innodb_trx r on r.trx_id = w.requesting_trx_id -> ) tmp, -> information_schema.processlist p, -> performance_schema.events_statements_current c, -> performance_schema.threads t -> where -> tmp.blocking_thread = p.id -> and t.thread_id = c.thread_id -> and t.processlist_id = p.id \g *************************** 1. row *************************** wating_trx_state: lock wait waiting_trx_id: 6180 waiting_thread: 19 waiting_query: update stu_tb set stu_name = 'wang' where stu_id = 1006 blocking_trx_state: running blocking_trx_id: 6177 blocking_thread: 20 blocking_query: null blocking_sql_text: select * from stu_tb where stu_id = 1006 for update blocking_host: localhost
上面结果显得更加清晰，我们可以清楚的看到阻塞端及被阻塞端事务执行的语句，有助于我们排查并确认是否可以杀掉阻塞的会话。
3.监控长事务
现实工作中我们需要监控下长事务，定义一个阈值，比如说30s 执行时间超过30s的事务即为长事务，要求记录并告警出来，提醒管理人员去处理。下面给出监控脚本，各位可以参考下，根据需求改动使用：
#!/bin/bash # ------------------------------------------------------------------------------- # filename: long_trx.sh # describe: monitor long transaction # revision: 1.0 # date: 2019/09/16 # author: wang /usr/local/mysql/bin/mysql -n -uroot -pxxxxxx -e "select now(),(unix_timestamp(now()) - unix_timestamp(a.trx_started)) diff_sec,b.id,b.user,b.host,b.db,d.sql_text from information_schema.innodb_trx a inner join information_schema.processlist b on a.trx_mysql_thread_id=b.id and b.command = 'sleep' inner join performance_schema.threads c on b.id = c.processlist_id inner join performance_schema.events_statements_current d on d.thread_id = c.thread_id;" | while read a b c d e f g h do if [ "$c" -gt 30 ] then echo $(date +"%y-%m-%d %h:%m:%s") echo "processid[$d] $e@$f in db[$g] hold transaction time $c sql:$h" fi done >> /tmp/longtransaction.txt
简单说明一下，这里的-gt 30是30秒钟的意思，只要超过了30秒钟就认定是长事务，可以根据实际需要自定义。将该脚本加入定时任务中即可执行。
总结：
本文主要介绍了长事务相关内容，怎样找到长事务，怎么处理长事务，如何监控长事务。可能有些小伙伴对事务理解还不多，希望这篇文章对你有所帮助。由于本篇文章列出的查询事务相关语句较多，现总结如下：
# 查询所有正在运行的事务及运行时间 select t.*,to_seconds(now())-to_seconds(t.trx_started) idle_time from information_schema.innodb_trx t \g # 查询事务详细信息及执行的sql select now(),(unix_timestamp(now()) - unix_timestamp(a.trx_started)) diff_sec,b.id,b.user,b.host,b.db,d.sql_text from information_schema.innodb_trx a inner join information_schema.processlist b on a.trx_mysql_thread_id=b.id and b.command = 'sleep' inner join performance_schema.threads c on b.id = c.processlist_id inner join performance_schema.events_statements_current d on d.thread_id = c.thread_id; # 查询事务执行过的所有历史sql记录 select ps.id 'process id', ps.user, ps.host, esh.event_id, trx.trx_started, esh.event_name 'event name', esh.sql_text 'sql', ps.time from performance_schema.events_statements_history esh join performance_schema.threads th on esh.thread_id = th.thread_id join information_schema.processlist ps on ps.id = th.processlist_id left join information_schema.innodb_trx trx on trx.trx_mysql_thread_id = ps.id where trx.trx_id is not null and ps.user != 'system_user' order by esh.event_id; # 简单查询事务锁 select * from sys.innodb_lock_waits\g # 查询事务锁详细信息 select tmp.*, c.sql_text blocking_sql_text, p.host blocking_host from ( select r.trx_state wating_trx_state, r.trx_id waiting_trx_id, r.trx_mysql_thread_id waiting_thread, r.trx_query waiting_query, b.trx_state blocking_trx_state, b.trx_id blocking_trx_id, b.trx_mysql_thread_id blocking_thread, b.trx_query blocking_query from information_schema.innodb_lock_waits w inner join information_schema.innodb_trx b on b.trx_id = w.blocking_trx_id inner join information_schema.innodb_trx r on r.trx_id = w.requesting_trx_id ) tmp, information_schema.processlist p, performance_schema.events_statements_current c, performance_schema.threads t where tmp.blocking_thread = p.id and t.thread_id = c.thread_id and t.processlist_id = p.id \g
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