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。 复制代码

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复制代码
在结果中idletime是计算产生的，也是事务的持续时间。但事务的trxquery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 | +---------------------+----------+----+------+-----------+--------+-----------------------------------------------------+复制代码
上述结果中diffsec和上面idletime表示意思相同，都是代表此事务持续的秒数。SQLTEXT表示该事务刚执行的SQL。但是呢，上述语句只能查到事务后执行的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eventsstatements_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 | +------------+------+-----------+----------+---------------------+------------------------------+-----------------------------------------------------+------+复制代码
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.innodblockwaits 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复制代码
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复制代码
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复制代码
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复制代码