前言：
本篇文章主要介紹MySQL長事務相關內容，比如說我們開啟的一個事務，一直沒提交或回滾會怎樣呢，出現事務等待情況應該如何處理，本篇文章將給你答案。
注意：本篇文章并不聚焦于談論事務隔離級別以及相關特性。而是介紹長事務相關危害以及監控處理方法。本文是基于MySQL5.7.23版本，不可重復讀(RR)隔離級別所做實驗。(語句為\G可以使查詢結構顯示更易讀，但只可以在mysql命令行使用。)
1.什么是長事務
首先我們先要知道什么是長事務，顧名思義就是運行時間比較長，長時間未提交的事務，也可以稱之為大事務。這類事務往往會造成大量的阻塞和鎖超時，容易造成主從延遲，要盡量避免使用長事務。
下面我將演示下如何開啟事務及模擬長事務：
#假設我們有一張stu_tb表，結構及數據如下
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 '自增主鍵',
`stu_id` int(11) NOT NULL COMMENT '學號',
`stu_name` varchar(20) DEFAULT NULL COMMENT '學生姓名',
`create_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT '創建時間',
`update_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP COMMENT '修改時間',
PRIMARY KEY (`increment_id`),
UNIQUE KEY `uk_stu_id` (`stu_id`) USING BTREE
) ENGINE=InnoDB AUTO_INCREMENT=9 DEFAULT CHARSET=utf8 COMMENT='測試學生表'
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)
#顯式開啟事務，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)
#如果我們不及時提交上個事務，那么這個事務就變成了長事務，當其他會話要操作這條數據時，就會一直等待。
2.如何找到長事務
遇到事務等待問題時，我們首先要做的是找到正在執行的事務。information_schema.INNODB_TRX 表中包含了當前innodb內部正在運行的事務信息，這個表中給出了事務的開始時間，我們可以稍加運算即可得到事務的運行時間。
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
在結果中idle_time是計算產生的，也是事務的持續時間。但事務的trx_query是NUL，這并不是說事務什么也沒執行，一個事務可能包含多個SQL，如果SQL執行完畢就不再顯示了。當前事務正在執行，innodb也不知道這個事務后續還有沒有sql,啥時候會commit。因此trx_query不能提供有意義的信息。
如果我們想看到這個事務執行過的SQL，看是否可以殺掉長事務，怎么辦呢？我們可以聯合其他系統表查詢得到，具體查詢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 |
+---------------------+----------+----+------+-----------+--------+-----------------------------------------------------+
上述結果中diff_sec和上面idle_time表示意思相同，都是代表此事務持續的秒數。SQL_TEXT表示該事務剛執行的SQL。但是呢，上述語句只能查到事務最后執行的SQL，我們知道，一個事務里可能包含多個SQL，那我們想查詢這個未提交的事務執行過哪些SQL，是否可以滿足呢，答案是結合events_statements_history系統表也可以滿足需求。下面語句將會查詢出該事務執行過的所有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 |
+------------+------+-----------+----------+---------------------+------------------------------+-----------------------------------------------------+------+
從上述結果中我們可以看到該事務從一開始到現在執行過的所有SQL，當我們把該事務相關信息都查詢清楚后，我們就可以判定該事務是否可以殺掉，以免影響其他事務造成等待現象。
在這里稍微拓展下，長事務極易造成阻塞或者死鎖現象，通常情況下我們可以首先查詢 sys.innodb_lock_waits 視圖確定有沒有事務阻塞現象：
#假設一個事務執行 select * from stu_tb where stu_id = 1006 for update
#另外一個事務執行 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
上述結果顯示出被阻塞的SQL以及鎖的類型，更強大的是殺掉會話的語句也給出來了。但是并沒有找到阻塞會話執行的SQL，如果我們想找出更詳細的信息，可以使用下面語句：
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
上面結果顯得更加清晰，我們可以清楚的看到阻塞端及被阻塞端事務執行的語句，有助于我們排查并確認是否可以殺掉阻塞的會話。
3.監控長事務
現實工作中我們需要監控下長事務，定義一個閾值，比如說30s 執行時間超過30s的事務即為長事務，要求記錄并告警出來，提醒管理人員去處理。下面給出監控腳本，各位可以參考下，根據需求改動使用：
#!/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
簡單說明一下，這里的-gt 30是30秒鐘的意思，只要超過了30秒鐘就認定是長事務，可以根據實際需要自定義。將該腳本加入定時任務中即可執行。
總結：
本文主要介紹了長事務相關內容，怎樣找到長事務，怎么處理長事務，如何監控長事務。可能有些小伙伴對事務理解還不多，希望這篇文章對你有所幫助。由于本篇文章列出的查詢事務相關語句較多，現總結如下：
# 查詢所有正在運行的事務及運行時間
select t.*,to_seconds(now())-to_seconds(t.trx_started) idle_time from INFORMATION_SCHEMA.INNODB_TRX t \G
# 查詢事務詳細信息及執行的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;
# 查詢事務執行過的所有歷史SQL記錄
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;
# 簡單查詢事務鎖
select * from sys.innodb_lock_waits\G
# 查詢事務鎖詳細信息
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
以上就是深入了解mysql長事務的詳細內容，更多關于mysql長事務的資料請關注腳本之家其它相關文章！
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