用MySQL慢日志分析解決MySQL CPU占用高的問題
首先找到MySQL的配置文件my.cnf，根據不同版本的mysql開啟慢查詢的配置也不一樣
mysql 5.0
[mysqld]
long_query_time = 1
log-slow-queries = /var/log/mysql/slow.log
mysql 5.1
[mysqld]
long_query_time = 1
slow_query_log=1
slow_query_log_file = /var/log/mysql/slow.log
long_query_time 是指執行超過多久的sql會被log下來，這里是1秒。
log-slow-queries和slow_query_log_file 設置把日志寫在哪里
把上述參數打開，運行一段時間，就可以關掉了，省得影響生產環境
接下來就是分析了，我這里的文件名字叫 /var/log/mysql/slow.log。
先mysqldumpslow –help下，主要用的是
-s ORDER what to sort by (t, at, l, al, r, ar etc), ‘at’ is default
-t NUM just show the top n queries
-g PATTERN grep: only consider stmts that include this string
-s，是order的順序，說明寫的不夠詳細，主要有
c,t,l,r和ac,at,al,ar，分別是按照query次數，時間，lock的時間和返回的記錄數來排序，前面加了a的時倒序
-t，是top n的意思，即為返回前面多少條的數據
-g，后邊可以寫一個正則匹配模式，大小寫不敏感的
mysqldumpslow -s c -t 20 /var/log/mysql/slow.log
mysqldumpslow -s r -t 20 /var/log/mysql/slow.log
上述命令可以看出訪問次數最多的20個sql語句和返回記錄集最多的20個sql。
mysqldumpslow -t 10 -s t -g “left join” /var/log/mysql/slow.log
這個是按照時間返回前10條里面含有左連接的sql語句。
例子,正確安全清空在線慢查詢日志slow log的流程
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>
[[email protected] ~]$ more /mysqllog/slow_log/slow_queries_3306_new.log
......
Time Id Command Argument
# Time: 140213 6:44:24
# [email protected]: 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
用了這個工具就可以查詢出來那些sql語句是性能的瓶頸，進行優化，比如加索引，該應用的實現方式等
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