linux下mysql开启慢查询

mysql中最影响速度的就是那些查询很慢的语句。这些慢的语句，可能是写的不够合理或者是大数据下多表的联合查询等等。所以我们要找出这些语句，分析原因，加以优化。

1、方法1：用命令开启慢查询

    1）、查看默认慢查询的时间：

mysql> show variables like "%long%";
+--------------------+-----------+
| Variable_name      | Value     |
+--------------------+-----------+
| long_query_time    | 10.000000 |
| max_long_data_size | 1048576   |
+--------------------+-----------+
2 rows in set (0.24 sec)

mysql> set global long_query_time=2;
Query OK, 0 rows affected (0.25 sec)

    3）、查看慢查询是否开启

mysql> show variables like "%slow%";
+---------------------+-----------------------------------------+
| Variable_name       | Value                                   |
+---------------------+-----------------------------------------+
| log_slow_queries    | OFF                                     |
| slow_launch_time    | 2                                       |
| slow_query_log      | OFF                                     |
| slow_query_log_file | /usr/local/mysql/var/li411-195-slow.log |
+---------------------+-----------------------------------------+
4 rows in set (0.23 sec)

    4）、开启慢查询

mysql> set slow_query_log='ON';
ERROR 1229 (HY000): Variable 'slow_query_log' is a GLOBAL variable and should be
 set with SET GLOBAL

mysql> set global slow_query_log='ON';
Query OK, 0 rows affected (1.11 sec)

mysql> show variables like "%slow%";
+---------------------+-----------------------------------------+
| Variable_name       | Value                                   |
+---------------------+-----------------------------------------+
| log_slow_queries    | ON                                      |
| slow_launch_time    | 2                                       |
| slow_query_log      | ON                                      |
| slow_query_log_file | /usr/local/mysql/var/li411-195-slow.log |
+---------------------+-----------------------------------------+
4 rows in set (4.04 sec)

2、方法2：改动mysql配置文件my.cnf

long_query_time = 2
log-slow-queries = /usr/local/mysql/var/localhost-slow.log