Code summary of Mysql slow query operation

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Mysql slow query explanation

MySQL’s slow query log is a type provided by MySQL Logging, which is used to record statements whose response time exceeds the threshold in MySQL, specifically refers to SQL whose running time exceeds the long_query_time value, which will be recorded in the slow query log. The default value of long_query_time is 10, which means running statements for more than 10S. By default, Mysql database does not start the slow query log. We need to manually set this parameter. Of course, if it is not needed for tuning, it is generally not recommended to start this parameter, because turning on the slow query log will cause or More or less will have a certain performance impact. The slow query log supports writing log records to files and also supports writing log records to database tables; the slow query log is used to record some query statements that are too slow, which can help administrators analyze the problem. The log defaults to no To enable it, you need to add a series of parameters to the configuration file to start it manually

Reason for turning on Mysql slow query

The database is a place where bottlenecks can easily occur. Nowadays, Nosql is being discussed so much that everyone is probably depressed by the database. What most affects the speed of MySQL are those very slow query statements. These slow statements may be written unreasonably or are joint queries of multiple tables under big data, etc., so we need to find out these statement, analyze the reasons, and optimize them. This is also the reason for posting this blog post

Enable mysql slow query method

1) Method 1 , Log in to the mysql database terminal and open

mysql> show variables like "%long%";
<SPAN style="COLOR: #ff00ff"> //查看一下默认为慢查询的时间10秒 </SPAN>
+-----------------+-----------+
| Variable_name  | Value   |
+-----------------+-----------+
| long_query_time | 10.000000 |
+-----------------+-----------+
1 row in set (0.00 sec)

mysql> set global long_query_time=1;
<SPAN style="COLOR: #ff00ff">//设置成1秒，加上global,下次进mysql已然生效</SPAN>
Query OK, 0 rows affected (0.00 sec)

mysql> show variables like "%slow%";
<SPAN style="COLOR: #ff00ff"> //查看一下慢查询是不是已经开启 </SPAN>
+---------------------+---------------------------------+
| Variable_name    | Value              |
+---------------------+---------------------------------+
| log_slow_queries  | OFF               |
| slow_launch_time  | 2                |
| slow_query_log   | OFF               |
| slow_query_log_file | /usr/local/mysql/mysql-slow.log |
+---------------------+---------------------------------+
4 rows in set (0.00 sec)

mysql> set slow_query_log=&#39;ON&#39;;
<SPAN style="COLOR: #ff00ff"> //加上global，不然会报错的</SPAN>
ERROR 1229 (HY000): Variable &#39;slow_query_log&#39; is a GLOBAL variable and should be set with SET GLOBAL
mysql> set global slow_query_log=&#39;ON&#39;;
<SPAN style="COLOR: #ff00ff">//这个参数设置为ON，表示启用慢查询，可以捕获执行时间超过一定数值的SQL语句。</SPAN>
Query OK, 0 rows affected (0.28 sec)

mysql> show variables like "%slow%";
<SPAN style="COLOR: #ff00ff">//查看是否已经开启 </SPAN>
+---------------------+---------------------------------+
| Variable_name    | Value              |
+---------------------+---------------------------------+
| log_slow_queries  | ON               |
| slow_launch_time  | 2                |
| slow_query_log   | ON               |
| slow_query_log_file | /usr/local/mysql/mysql-slow.log |
+---------------------+---------------------------------+
4 rows in set (0.00 sec)

Method 2. Modify the mysql configuration file my.cnf

[root@www ~]# vim /etc/mysql.cnf
Add the following content in the [mysqld] area configuration
......

slow_query_log = 1

//开启慢查询日志,将1改为ON也可以
long_query_time = 1

//mysql慢查询时间，指定达到多少秒才算慢查询；这里推荐设置超过1秒的执行语句就会被记录到慢查询日志中
slow_query_log_file= /var/lib/mysql/mysql-slow.log

//慢查询日志路径。这里slow_query_log_file修改为log-slow-queries 也可以
//主要是上面三行内容，还可以添加下面几行细节性配置
long-queries-not-using-indexes

//记录没有使用索引的查询语句
min_examined_row_limit = 1000

 //记录查找了多达1000次而引起的慢查询
log-slow-admin-statements

  //记录ALTER TABLE等语句引发的慢查询
log-slow-slave-statements
 //记录从服务器产生的慢查询

Analysis Tool

The analysis tool is actually to analyze the data recorded in mysql-slow.log. It will be displayed at once (in fact, you can also get the required information out by writing a shell script yourself).

# [root@www ~]#Cat mysql-slow.log // View command

The execution status, including execution time, locking time, etc., so whether you want an analysis tool depends on your personal situation. There are many analysis tools. Here I will only talk about how to use the slow query analysis tool mysqldumpslow that comes with mysql.

[root@www ~]# mysqldumpslow -h

/usr/local/mysql/libexec/mysqld, Version: 5.1.26-rc-log (Source distribution). started with:
Tcp port: 3306 Unix socket: /tmp/mysql.sock
Time         Id Command  Argument
# Time: 100814 13:28:30
# User@Host: root[root] @ localhost []
# Query_time: 10.096500 Lock_time: 0.045791 Rows_sent: 1 Rows_examined: 2374192
SET timestamp=1281763710;
select count(distinct ad_code) as x from ad_visit_history where ad_code in (select ad_code from ad_list where media_id=15);
# Time: 100814 13:37:02
# User@Host: root[root] @ localhost []
# Query_time: 10.394134 Lock_time: 0.000091 Rows_sent: 1 Rows_examined: 2374192
SET timestamp=1281764222;
select count(distinct ad_code) as x from ad_visit_history where ad_code in (select ad_code from ad_list where media_id=15);
# Time: 100814 13:37:16
# User@Host: root[root] @ localhost []
# Query_time: 4.608920 Lock_time: 0.000078 Rows_sent: 1 Rows_examined: 1260544
SET timestamp=1281764236;
select count(*) as cou from ad_visit_history where ad_code in (select ad_code from ad_list where id=41) order by id desc;

Example:

Option h requires an argument
ERROR: bad option

Usage: mysqldumpslow [ OPTS... ] [ LOGS... ]

Parse and summarize the MySQL slow query log. Options are

 --verbose  verbose
 --debug   debug
 --help    write this text to standard output

 -v      verbose
 -d      debug     //查错
 -s ORDER   what to sort by (t, at, l, al, r, ar etc), 'at' is default   //排序方式query次数，时间，lock的时间和返回的记录数来排序
 -r      reverse the sort order (largest last instead of first)    //倒排序
 -t NUM    just show the top n queries                    //显示前N多个
 -a      don't abstract all numbers to N and strings to 'S'
 -n NUM    abstract numbers with at least n digits within names  //抽象的数字，至 少有n位内的名称
 -g PATTERN  grep: only consider stmts that include this string   //配置模式
 -h HOSTNAME hostname of db server for *-slow.log filename (can be wildcard),   //mysql所以机器名或者IP
 default is '*', i.e. match all
 -i NAME   name of server instance (if using mysql.server startup script)
 -l      don't subtract lock time from total time      //总时间中不减去锁定时间

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