mysql---慢查询

mysql有些sql会执行很慢，有可能造成服务器负载飙升

首先查询 确定影响负载的是mysql ，使用top命令，ps命令等

其次，进入MySQL，使用show full processlist查询执行中的sql语句，看看问题，使用explain 命令 查看状态

最后找出sql语句杀死或者优化

centos7上面安装mariadb服务

yum -y install mariadb-server mariadb-devel

开启慢查询

more /etc/my.cnf.d/server.cnf 

[mariadb]
slow_query_log=ON
slow_query_log_file=/usr/local/mysql/data/slow.log
long_query_time=1

启动mariadb服务

systemctl start mariadb

查询mysql的慢查询是否开启，以及多久的时间以上是慢查询

MariaDB [(none)]> show variables like '%slow_query%';
+---------------------+--------------------------------+
| Variable_name       | Value                          |
+---------------------+--------------------------------+
| slow_query_log      | ON                             |
| slow_query_log_file | /usr/local/mysql/data/slow.log |
+---------------------+--------------------------------+
2 rows in set (0.00 sec)


MariaDB [(none)]> show variables like 'long_query_time';
+-----------------+----------+
| Variable_name   | Value    |
+-----------------+----------+
| long_query_time | 1.000000 |
+-----------------+----------+
1 row in set (0.00 sec)

#如果没用开启慢查询，可以在命令行开启
mysql> set global slow_query_log=1;
Query OK, 0 rows affected (0.00 sec)

 测试慢查询，以及查看日志

MariaDB [(none)]> select sleep(2);
+----------+
| sleep(2) |
+----------+
|        0 |
+----------+
1 row in set (2.00 sec)

[root@localhost ~]# more /usr/local/mysql/data/slow.log
/usr/libexec/mysqld, Version: 5.5.60-MariaDB (MariaDB Server). started with:
Tcp port: 0  Unix socket: /var/lib/mysql/mysql.sock
Time                 Id Command    Argument
# Time: 180930 23:51:07
# User@Host: root[root] @ localhost []
# Thread_id: 2  Schema:   QC_hit: No
# Query_time: 2.001017  Lock_time: 0.000000  Rows_sent: 1  Rows_examined: 0
SET timestamp=1538322667;
select sleep(2);

 确认慢查询

MariaDB [(none)]> show full processlist;  #查看state慢查询在进行
+----+------+-----------+------+---------+------+------------+-----------------------+----------+
| Id | User | Host      | db   | Command | Time | State      | Info                  | Progress |
+----+------+-----------+------+---------+------+------------+-----------------------+----------+
|  3 | root | localhost | NULL | Query   |    9 | User sleep | select sleep(10)      |    0.000 |
|  4 | root | localhost | NULL | Query   |    0 | NULL       | show full processlist |    0.000 |
+----+------+-----------+------+---------+------+------------+-----------------------+----------+
2 rows in set (0.00 sec)

MariaDB [(none)]> show full processlist; #查看state慢查询已经结束，但是用户登陆了
+----+------+-----------+------+---------+------+-------+-----------------------+----------+
| Id | User | Host      | db   | Command | Time | State | Info                  | Progress |
+----+------+-----------+------+---------+------+-------+-----------------------+----------+
|  3 | root | localhost | NULL | Sleep   |    1 |       | NULL                  |    0.000 |
|  4 | root | localhost | NULL | Query   |    0 | NULL  | show full processlist |    0.000 |
+----+------+-----------+------+---------+------+-------+-----------------------+----------+
2 rows in set (0.00 sec)