How to quickly locate slow SQL in MySQL

Open slow query log

We often encounter slow queries in the project. When we encounter slow queries, we generally need to enable the slow query log, and analyze the slow query log to find the slow SQL. Then use explain to analyze

System variables

The system variables related to MySQL and slow query are as follows

Execute the following statement to see whether the slow query log is enabled. ON means enabled, OFF means not enabled.

show variables like "%slow_query_log%"

You can see that mine is not enabled. You can use the following two methods. Method to enable slow query

Modify the configuration file

Modify the configuration file my.ini and add the following parameters in the [mysqld] paragraph

[mysqld]
log_output='FILE,TABLE'
slow_query_log='ON'
long_query_time=0.001

You need to restart MySQL. It takes effect, the command is service mysqld restart

Set global variables

I execute the following 2 sentences on the command line to open the slow query log, set the timeout to 0.001s, and record the log Go to the file and the mysql.slow_log table

set global slow_query_log = on;
set global log_output = 'FILE,TABLE';
set global long_query_time = 0.001;

If you want to make it permanent, get the configuration in the configuration file, otherwise these configurations will become invalid after the database is restarted

Analyze the slow query log

Because the mysql slow query log is equivalent to a running account and does not have the function of summary statistics, we need to use some tools to analyze it

mysqldumpslow

mysql has a built-in mysqldumpslow tool Come help us analyze slow query logs.

Common Usage

# 取出使用最多的10条慢查询
mysqldumpslow -s c -t 10 /var/run/mysqld/mysqld-slow.log

# 取出查询时间最慢的3条慢查询
mysqldumpslow -s t -t 3 /var/run/mysqld/mysqld-slow.log 

# 得到按照时间排序的前10条里面含有左连接的查询语句
mysqldumpslow -s t -t 10 -g “left join” /database/mysql/slow-log

pt-query-digest

pt-query-digest is the tool I use most, its functions It is very powerful and can analyze binlog, general log, slowlog, and can also be analyzed through show processlist or MySQL protocol data captured through tcpdump. Just download and authorize it to run the pt-query-digest Perl script

Download and empowerment

wget www.percona.com/get/pt-query-digest
chmod u+x pt-query-digest
ln -s /opt/soft/pt-query-digest /usr/bin/pt-query-digest

Usage introduction

// 查看具体使用方法 
pt-query-digest --help
// 使用格式
pt-query-digest [OPTIONS] [FILES] [DSN]

COMMON OPTIONS

• --create-review-table When using the --review parameter to output the analysis results to the table, it will be automatically created if there is no table.

• --create-history-table When using the --history parameter to output the analysis results to a table, it will be automatically created if there is no table.

• --filter Matches and filters the input slow query according to the specified string and then analyzes it

• --limit limits the output results Percentage or quantity, the default value is 20, that is, the 20 slowest statements will be output. If it is 50%, it will be sorted from large to small by the total response time, and the output will be cut off when the total reaches 50%.

• --host mysql server address

• --user mysql username

• -- password mysql user password

• --history Save the analysis results to the table. The analysis results are more detailed. The next time you use --history, if the same statement exists and the query is If the time interval is different from that in the history table, it will be recorded in the data table. You can compare the historical changes of a certain type of query by querying the same CHECKSUM.

• --review Save the analysis results to the table. This analysis only parameterizes the query conditions. One type of query is for one record, which is relatively simple. If the same statement analysis occurs, it will not be recorded in the data table the next time --review is used.

• --output analysis result output type, the value can be report (standard analysis report), slowlog (Mysql slow log), json, json-anon, generally use report for easier reading .

• --since the time from which to start analysis, the value is a string, which can be a specified time point in the format of "yyyy-mm-dd [hh:mm:ss]" , or it can be a simple time value: s (seconds), h (hours), m (minutes), d (days), for example, 12h means that statistics started 12 hours ago.

• --until deadline, combined with -since can analyze slow queries within a period of time.

Commonly used DSN

A Specify the character set
D Specify the connected database
P Connect to the database port
S Connect Socket file
h Host name to connect to the database
p Password to connect to the database
t Which table should the data be stored in when using --review or --history
u User name to connect to the database

DSN is configured in the form of key=value; multiple DSNs are used, separated

Usage example

# 展示slow.log中最慢的查询的报表
pt-query-digest slow.log

# 分析最近12小时内的查询
pt-query-digest --since=12h slow.log

# 分析指定范围内的查询
pt-query-digest slow.log --since '2020-06-20 00:00:00' --until '2020-06-25 00:00:00'

# 把slow.log中查询保存到query_history表
pt-query-digest --user=root --password=root123 --review h=localhost,D=test,t=query_history --create-review-table slow.log

# 连上localhost，并读取processlist，输出到slowlog
pt-query-digest --processlist h=localhost --user=root --password=root123 --interval=0.01 --output slowlog

# 利用tcpdump获取MySQL协议数据，然后产生最慢查询的报表
# tcpdump使用说明：https://blog.csdn.net/chinaltx/article/details/87469933
tcpdump -s 65535 -x -nn -q -tttt -i any -c 1000 port 3306 > mysql.tcp.txt
pt-query-digest --type tcpdump mysql.tcp.txt

# 分析binlog
mysqlbinlog mysql-bin.000093 > mysql-bin000093.sql
pt-query-digest  --type=binlog mysql-bin000093.sql

# 分析general log
pt-query-digest  --type=genlog  localhost.log

Practical usage

Writing stored procedures to create data in batches

There is no performance test in actual work. We often need to transform large batches of data, and manual insertion is impossible. At this time, we have to use stored procedures

CREATE TABLE `kf_user_info` (
  `id` int(11) NOT NULL COMMENT '用户id',
  `gid` int(11) NOT NULL COMMENT '客服组id',
  `name` varchar(25) NOT NULL COMMENT '客服名字'
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COMMENT='客户信息表';

How to define a stored procedure?

CREATE PROCEDURE 存储过程名称 ([参数列表])
BEGIN
    需要执行的语句
END

For example, insert 100000 pieces of data with IDs 1-100000

Use Navicat to execute

-- 删除之前定义的
DROP PROCEDURE IF EXISTS create_kf;

-- 开始定义
CREATE PROCEDURE create_kf(IN loop_times INT) 
BEGIN
	DECLARE var INT;
	SET var = 1;
	WHILE var < loop_times DO    
		INSERT INTO kf_user_info (`id`,`gid`,`name`) 
		VALUES (var, 1000, var);
		SET var = var + 1;
	END WHILE; 
END;

-- 调用
call create_kf(100000);

Three parameter types of the stored procedure

用MySQL执行

得用DELIMITER 定义新的结束符，因为默认情况下SQL采用（;）作为结束符，这样当存储过程中的每一句SQL结束之后，采用（；）作为结束符，就相当于告诉MySQL可以执行这一句了。但是存储过程是一个整体，我们不希望SQL逐条执行，而是采用存储过程整段执行的方式，因此我们就需要定义新的DELIMITER ，新的结束符可以用（//）或者（$$）

因为上面的代码应该就改为如下这种方式

DELIMITER //
CREATE PROCEDURE create_kf_kfGroup(IN loop_times INT)  
BEGIN  
	DECLARE var INT;
	SET var = 1;
	WHILE var <= loop_times DO    
		INSERT INTO kf_user_info (`id`,`gid`,`name`) 
		VALUES (var, 1000, var);
		SET var = var + 1;
	END WHILE;  
END //
DELIMITER ;

查询已经定义的存储过程

show procedure status;

开始执行慢sql

select * from kf_user_info where id = 9999;
select * from kf_user_info where id = 99999;
update kf_user_info set gid = 2000 where id = 8888;
update kf_user_info set gid = 2000 where id = 88888;

可以执行如下sql查看慢sql的相关信息。

SELECT * FROM mysql.slow_log order by start_time desc;

查看一下慢日志存储位置

show variables like "slow_query_log_file"

pt-query-digest /var/lib/mysql/VM-0-14-centos-slow.log

执行后的文件如下

# Profile
# Rank Query ID                            Response time Calls R/Call V/M 
# ==== =================================== ============= ===== ====== ====
#    1 0xE2566F6154AFF41948FE497E53631B43   0.1480 56.1%     4 0.0370  0.00 UPDATE kf_user_info
#    2 0x2DFBC6DBF0D68EF2EC2AE954DC37A1A4   0.1109 42.1%     4 0.0277  0.00 SELECT kf_user_info
# MISC 0xMISC                               0.0047  1.8%     2 0.0024   0.0 <2 ITEMS>

从最上面的统计sql中就可以看到执行慢的sql

可以看到响应时间，执行次数，每次执行耗时（单位秒），执行的sql

下面就是各个慢sql的详细分析，比如，执行时间，获取锁的时间，执行时间分布，所在的表等信息

不由得感叹一声，真是神器，查看慢sql超级方便

最后说一个我遇到的一个有意思的问题，有一段时间线上的接口特别慢，但是我查日志发现sql执行的很快，难道是网络的问题？

为了确定是否是网络的问题，我就用拦截器看了一下接口的执行时间，发现耗时很长，考虑到方法加了事务，难道是事务提交很慢？

于是我用pt-query-digest统计了一下1分钟左右的慢日志，发现事务提交的次很多，但是每次提交事务的平均时长是1.4s左右，果然是事务提交很慢。

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