MySQL-長事務詳解
阿新 • • 發佈：2019-12-31
前言：
『入門MySQL』系列文章已經完結，今後我的文章還是會以MySQL為主，主要記錄下近期工作及學習遇到的場景或者自己的感悟想法，可能後續的文章不是那麼連貫，但還是希望大家多多支援。言歸正傳，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：本篇文章並不聚焦於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。本文是基於MySQL5.7.23版本，不可重複讀(RR)隔離級別所做實驗。
1.什麼是長事務
首先我們先要知道什麼是長事務，顧名思義就是執行時間比較長，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，要儘量避免使用長事務。
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，結構及資料如下
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 '建立時間',`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，那麼這個事務就變成了長事務，當其他會話要操作這條資料時，就會一直等待。
複製程式碼 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內部正在執行的事務資訊，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執行時間。
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複製程式碼 在結果中idletime是計算產生的，也是事務的持續時間。但事務的trxquery是NUL，這並不是說事務什麼也沒執行，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也不知道這個事務後續還有沒有sql,啥時候會commit。 因此trx_query不能提供有意義的資訊。
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，怎麼辦呢？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 |
+---------------------+----------+----+------+-----------+--------+-----------------------------------------------------+複製程式碼上述結果中diffsec和上面idletime表示意思相同，都是代表此事務持續的秒數。SQLTEXT表示該事務剛執行的SQL。但是呢，上述語句只能查到事務最後執行的SQL，我們知道，一個事務裡可能包含多個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eventsstatements_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 |
+------------+------+-----------+----------+---------------------+------------------------------+-----------------------------------------------------+------+複製程式碼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資訊都查詢清楚後，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。
在這裡稍微拓展下，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.innodblockwaits 檢視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複製程式碼上述結果顯示出被阻塞的SQL以及鎖的型別，更強大的是殺掉會話的語句也給出來了。但是並沒有找到阻塞會話執行的SQL，如果我們想找出更詳細的資訊，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複製程式碼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，有助於我們排查並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，要求記錄並告警出來，提醒管理人員去處理。下面給出監控指令碼，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(),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複製程式碼簡單說明一下，這裡的-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。將該指令碼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，怎麼處理長事務，如何監控長事務。可能有些小夥伴對事務理解還不多，希望這篇文章對你有所幫助。由於本篇文章列出的查詢事務相關語句較多，現總結如下：
# 查詢所有正在執行的事務及執行時間
select t.*,to_seconds(now())-to_seconds(t.trx_started) idle_time from INFORMATION_SCHEMA.INNODB_TRX t \G
# 查詢事務詳細資訊及執行的SQL
select now(),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
# 查詢事務鎖詳細資訊
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複製程式碼