MySQL-长事务详解

前言：

『入门MySQL』系列文章已经完结，今后我的文章还是会以MySQL为主，主要记录下近期工作及学习遇到的场景或者自己的感悟想法，可能后续的文章不是那么连贯，但还是希望大家多多支持。言归正传，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。

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