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。

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_tbCreate 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 transactionmysql> 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。

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: NULLtrx_wait_started: NULLtrx_weight: 3 trx_mysql_thread_id: 11 trx_query: NULL trx_operation_state: NULL trx_tables_in_use: 0 trx_tables_locked: 1trx_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: 1trx_foreign_key_checks: 1trx_last_foreign_key_error: NULL trx_adaptive_hash_latched: 0 trx_adaptive_hash_timeout: 0trx_is_read_only: 0trx_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 = 1006mysql> select * from sys.innodb_lock_waits\G*************************** 1. row ***************************wait_started: 2019-09-16 14:34:32wait_age: 00:00:03 wait_age_secs: 3locked_table: `testdb`.`stu_tb`locked_index: uk_stu_id locked_type: RECORDwaiting_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: 6177blocking_pid: 20blocking_query: NULLblocking_lock_id: 6177:47:4:7blocking_lock_mode: Xblocking_trx_started: 2019-09-16 14:18:44blocking_trx_age: 00:15:51blocking_trx_rows_locked: 2blocking_trx_rows_modified: 0 sql_kill_blocking_query: KILL QUERY 20sql_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 WAITwaiting_trx_id: 6180waiting_thread: 19 waiting_query: update stu_tb set stu_name = 'wang' where stu_id = 1006blocking_trx_state: RUNNING blocking_trx_id: 6177 blocking_thread: 20blocking_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。

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 joininformation_schema.PROCESSLIST bon a.TRX_MYSQL_THREAD_ID=b.id and b.command = 'Sleep'inner join performance_schema.threads c ON b.id = c.PROCESSLIST_IDinner join performance_schema.events_statements_current d ON d.THREAD_ID = c.THREAD_ID;" | while read A B C D E F G Hdoif [ "$C" -gt 30 ]thenecho $(date +"%Y-%m-%d %H:%M:%S")echo "processid[$D] $E@$F in db[$G] hold transaction time $C SQL:$H"fidone >> /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SQLselect 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 bon a.TRX_MYSQL_THREAD_ID=b.id and b.command = 'Sleep'inner join performance_schema.threads c ON b.id = c.PROCESSLIST_IDinner 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SELECTps.id 'PROCESS ID',ps.USER,ps.HOST,esh.EVENT_ID,trx.trx_started,esh.event_name 'EVENT NAME',esh.sql_text 'SQL',ps.time FROMPERFORMANCE_SCHEMA.events_statements_history eshJOIN PERFORMANCE_SCHEMA.threads th ON esh.thread_id = th.thread_idJOIN information_schema.PROCESSLIST ps ON ps.id = th.processlist_idLEFT JOIN information_schema.innodb_trx trx ON trx.trx_mysql_thread_id = ps.id WHEREtrx.trx_id IS NOT NULL AND ps.USER != 'SYSTEM_USER' ORDER BYesh.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 SELECTtmp.*,c.SQL_Text blocking_sql_text,p.HOST blocking_host FROM(SELECTr.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 FROMinformation_schema.innodb_lock_waits wINNER JOIN information_schema.innodb_trx b ON b.trx_id = w.blocking_trx_idINNER 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 WHEREtmp.blocking_thread = p.id AND t.thread_id = c.THREAD_ID AND t.PROCESSLIST_ID = p.id \G