MySQL 慢查詢檔案清空教程

配置慢查詢

預設的my.cnf檔案在/etc/目錄下

vim /etc/my.cnf

#slow_query

slow_query_log=1#開啟慢查詢

slow_query_log_file=/var/lib/mysql/mysql-slow.log #設定慢查詢路徑

long_query_time=0.01#設定慢查詢時間

配置完要重啟mysql服務才生效

查詢慢查詢配置是否成功

測試慢查詢

use information_schema;

select * from tables;

語句執行時間花了：1.1秒，返回了314行記錄;

查看慢查詢檔案：cat mysql-slow.log

慢查詢記錄檔中記錄了每一天慢查詢記錄，每一條記錄都是以Time:開頭;記錄了包含登入資訊，查詢所花的時間，鎖花的時間，返回的行數，掃描的記錄數，執行的語句。

線上產生慢查詢記錄檔

1.關閉全域慢查詢

SET global slow_query_log=0

查詢是否關閉全域慢查詢

SHOW VARIABLES LIKE '%query_log%';

設定一個新的慢查詢檔案

SET global slow_query_log_file='/var/lib/mysql/mysql-slow_new.log'

開啟慢查詢

SET global slow_query_log=1;

SHOW VARIABLES LIKE '%query_log%';

在慢查詢路徑中可以看到產生了一個新的慢查詢檔案。

後面的慢查詢資訊會記錄到新的記錄檔當中，

這時我們可以mv以前的慢查詢檔案進行歸檔了。

重啟mysql服務

service mysql restart

SHOW VARIABLES LIKE '%query_log%';

當重啟mysql服務之後，慢記錄檔會重新變成之前的在my.cnf裡面設定的檔案，所以如果你想要重啟服務之後慢記錄檔還是剛才設定的 mysql-slow_new.log的話你就需要在修改全域設定的同時再修改my.cnf檔案，這樣就能保證重啟之後檔案還是之前修改的檔案。

總結

配置慢查詢對平時收集效能差的語句很有協助，也有很多工具專門來分析慢查詢日誌的，其中percona-toolkit就是一個很不錯的分析慢查詢的工具，如果沒有使用可以去瞭解一下。

mysql正確安全清空線上慢查詢日誌slow log


1, see the slow log status;

mysql> show variables like '%slow%';
+---------------------+------------------------------------------+
| Variable_name       | Value                                    |
+---------------------+------------------------------------------+
| log_slow_queries    | ON                                       |
| slow_launch_time    | 2                                        |
| slow_query_log      | ON                                       |
| slow_query_log_file | /mysqllog/slow_log/slow_queries_3306.log |
+---------------------+------------------------------------------+
4 rows in set (0.00 sec)

2, stop the slow log server.

mysql> set global slow_query_log=0;
Query OK, 0 rows affected (0.27 sec)

mysql> show variables like '%slow%';
+---------------------+------------------------------------------+
| Variable_name       | Value                                    |
+---------------------+------------------------------------------+
| log_slow_queries    | OFF                                      |
| slow_launch_time    | 2                                        |
| slow_query_log      | OFF                                      |
| slow_query_log_file | /mysqllog/slow_log/slow_queries_3306.log |
+---------------------+------------------------------------------+
4 rows in set (0.00 sec)

mysql>
mysql> show variables like '%slow%'; -- check slow log status
+---------------------+------------------------------------------+
| Variable_name       | Value                                    |
+---------------------+------------------------------------------+
| log_slow_queries    | OFF                                      |
| slow_launch_time    | 2                                        |
| slow_query_log      | OFF                                      |
| slow_query_log_file | /mysqllog/slow_log/slow_queries_3306.log |
+---------------------+------------------------------------------+
4 rows in set (0.00 sec)

3, reset the new path of slow log

mysql> set global slow_query_log_file='/mysqllog/slow_log/slow_queries_3306_new.log';
Query OK, 0 rows affected (0.03 sec)

4, start the slow log server

mysql>
mysql>
mysql> set global slow_query_log=1;
Query OK, 0 rows affected (0.01 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 | /mysqllog/slow_log/slow_queries_3306_new.log |
+---------------------+----------------------------------------------+
4 rows in set (0.00 sec)

5, check the slow sql in the new slow log file.

mysql> select sleep(10) as a, 1 as b;
+---+---+
| a | b |
+---+---+
| 0 | 1 |
+---+---+
1 row in set (10.00 sec)

mysql>
[mysql@xxx-xxx ~]$ more /mysqllog/slow_log/slow_queries_3306_new.log
......
Time                 Id Command    Argument
# Time: 140213  6:44:24
# User@Host: root[root] @ localhost []
# Query_time: 10.000365  Lock_time: 0.000000 Rows_sent: 1  Rows_examined: 0
SET timestamp=1392273864;
select sleep(10) as a, 1 as b;

6, backup the old big slow log file to other directory.

mv /mysqllog/slow_log/slow_queries_3306.log /mysqlbackup/slow_log/slow_queries_3306.log.bak.20140213