MySQL-长事务详解

2019 年 10 月 6 日
筆記

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。(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_tbG  *************************** 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_waitsG  *************************** 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_waitsG     # 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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