MySQL 慢查询

什么是慢查询

　　就是很慢的查询啦，那多慢才算慢呢？这个你说了算，你要是觉得0.000000000000秒都算慢，那么就是说，你将每一条查询都认为是慢查询。

慢查询有什么用呢

　　没啥用，如果没有慢查询才好呢

　　有慢查询的时候，证明该优化数据库或者查询语句了，只是一个相当于日志的概念，没事找事。

　　

查询相关项的开启状态

#查看是否开启慢查询日志
mysql> show variables like 'slow_query_log';
+----------------+-------+
| Variable_name  | Value |
+----------------+-------+
| slow_query_log | OFF   |
+----------------+-------+
1 row in set (0.06 sec)

#查看是否开启没有使用索引的查询
mysql> show variables like '%log_qu%';
+-------------------------------+-------+
| Variable_name                 | Value |
+-------------------------------+-------+
| log_queries_not_using_indexes | OFF   |
+-------------------------------+-------+
1 row in set (0.00 sec)

#一个查询超过多少秒会被记录到慢查询日志中（如果是0，则表示每条命令都要记录）
mysql> show variables like '%long_query%';
+-----------------+-----------+
| Variable_name   | Value     |
+-----------------+-----------+
| long_query_time | 10.000000 |
+-----------------+-----------+
1 row in set (0.01 sec)

　　

开启慢查询

#开启慢查询
mysql> set global slow_query_log=ON;
Query OK, 0 rows affected (0.07 sec)

#查看慢查询状态、以及慢查询的日志文件
mysql> show variables like 'slow_query%';
+---------------------+------------------------------------------+
| Variable_name       | Value                                    |
+---------------------+------------------------------------------+
| slow_query_log      | ON                                       |
| slow_query_log_file | /usr/local/mysql/data/localhost-slow.log |
+---------------------+------------------------------------------+
2 rows in set (0.00 sec)

#设置查询超时时间，此处设为0，表示将每一条命令都记录到慢查询日志中
mysql> set @@long_query_time = 0.000000;
Query OK, 0 rows affected (0.00 sec)

mysql> show variables like '%long_query%';
+-----------------+----------+
| Variable_name   | Value    |
+-----------------+----------+
| long_query_time | 0.000000 |
+-----------------+----------+
1 row in set (0.00 sec)

mysql> set global log_queries_not_using_indexes=on;
Query OK, 0 rows affected (0.00 sec)

mysql> show variables like 'log_queries_not_using_indexes';
+-------------------------------+-------+
| Variable_name                 | Value |
+-------------------------------+-------+
| log_queries_not_using_indexes | ON    |
+-------------------------------+-------+
1 row in set (0.00 sec)

　　

测试

mysql> select * from category left join goods on category.id=goods.cate_id order by category.id;
+----+---------+------+---------+-------------------+
| id | cate    | id   | cate_id | name              |
+----+---------+------+---------+-------------------+
|  1 | food    |    2 |       1 | water             |
|  1 | food    |    3 |       1 | rice              |
|  2 | clothes |    1 |       2 | T-shirt           |
|  3 | book    |    4 |       3 | C++ primer        |
|  4 | sport   |    5 |       4 | basketbal         |
|  5 | music   |    0 |       5 | You Are Not Alone |
|  6 | video   | NULL |    NULL | NULL              |
+----+---------+------+---------+-------------------+
7 rows in set (0.00 sec)

#查看慢查询日志
localhost:~ root# cat /usr/local/mysql/data/localhost-slow.log

# Time: 180730 15:54:14
# User@Host: root[root] @ localhost []  Id:    10
# Query_time: 0.000582  Lock_time: 0.000181 Rows_sent: 7  Rows_examined: 20
SET timestamp=1532937254;
select * from category left join goods on category.id=goods.cate_id order by category.id;

使用mysqldumpslow

localhost:~ root# mysqldumpslow -t 10 /usr/local/mysql/data/localhost-slow.log

Reading mysql slow query log from /usr/local/mysql/data/localhost-slow.log
Count: 10  Time=0.00s (0s)  Lock=0.00s (0s)  Rows=7.0 (70), root[root]@localhost
  select * from category left join goods on category.id=goods.cate_id order by category.id