mysql 慢查詢

一，爲何要開啓這個查詢呢？

數據庫是很容易產生瓶頸的地方，如今Nosql你們討論這麼熱，估計都被數據庫搞鬱悶了。mysql中最影響速度的就是那些查詢很是慢的語句，這些慢的語句，多是寫的不夠合理或者是大數據下多表的聯合查詢等等，因此咱們要找出這些語句，分析緣由，加以優化。這也是發這篇博文的緣由

二，開啓mysql的慢查詢

方法1，用命令開啓慢查詢

mysql> show variables like "%long%";         //查看一下默認爲慢查詢的時間10秒
+-----------------+-----------+
| Variable_name   | Value     |
+-----------------+-----------+
| long_query_time | 10.000000 |
+-----------------+-----------+
1 row in set (0.00 sec)

mysql> set global long_query_time=2;          //設置成2秒，加上global,下次進mysql已然生效
Query OK, 0 rows affected (0.00 sec)

mysql> show variables like "%slow%";          //查看一下慢查詢是否是已經開啓
+---------------------+---------------------------------+
| Variable_name       | Value                           |
+---------------------+---------------------------------+
| log_slow_queries    | OFF                             |
| slow_launch_time    | 2                               |
| slow_query_log      | OFF                             |
| slow_query_log_file | /usr/local/mysql/mysql-slow.log |
+---------------------+---------------------------------+
4 rows in set (0.00 sec)

mysql> set slow_query_log='ON';                        //加上global，否則會報錯的。
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';            //啓用慢查詢
Query OK, 0 rows affected (0.28 sec)

mysql> show variables like "%slow%";              //查看是否已經開啓
+---------------------+---------------------------------+
| Variable_name       | Value                           |
+---------------------+---------------------------------+
| log_slow_queries    | ON                              |
| slow_launch_time    | 2                               |
| slow_query_log      | ON                              |
| slow_query_log_file | /usr/local/mysql/mysql-slow.log |
+---------------------+---------------------------------+
4 rows in set (0.00 sec)

方法2,修改mysql的配置文件my.cnf

在[mysqld]裏面加上如下內容

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

重起一下
/usr/local/mysql/libexec/mysqld restart

三，分析工具

分析工具幹什麼事的呢，其實就是把mysql-slow.log裏面記錄下來的數據，分析一下顯示出來。其實本身寫一個shell腳本也是能夠把要的信息取出來的。咱們來看一下mysql-slow.log裏面究竟是什麼東西

[root@BlackGhost mysql]# cat mysql-slow.log     //查看命令
/usr/local/mysql/libexec/mysqld, Version: 5.1.26-rc-log (Source distribution). started with:
Tcp port: 3306  Unix socket: /tmp/mysql.sock
Time                 Id Command    Argument
# Time: 100814 13:28:30
# User@Host: root[root] @ localhost []
# Query_time: 10.096500  Lock_time: 0.045791 Rows_sent: 1  Rows_examined: 2374192
SET timestamp=1281763710;
select count(distinct ad_code) as x from ad_visit_history where ad_code in (select ad_code from ad_list where media_id=15);
# Time: 100814 13:37:02
# User@Host: root[root] @ localhost []
# Query_time: 10.394134  Lock_time: 0.000091 Rows_sent: 1  Rows_examined: 2374192
SET timestamp=1281764222;
select count(distinct ad_code) as x from ad_visit_history where ad_code in (select ad_code from ad_list where media_id=15);
# Time: 100814 13:37:16
# User@Host: root[root] @ localhost []
# Query_time: 4.608920  Lock_time: 0.000078 Rows_sent: 1  Rows_examined: 1260544
SET timestamp=1281764236;
select count(*) as cou  from ad_visit_history where ad_code in (select ad_code from ad_list where id=41) order by id desc;

看到了，就是記錄一下sql語句的執行狀況，包括執行時間，鎖定時間等，因此要不要分析工具看我的狀況，分析工具不少，在這兒只說一下mysql自帶的慢查詢分析工具mysqldumpslow的使用方法。

[root@BlackGhost bin]# mysqldumpslow -h
Option h requires an argument
ERROR: bad option

Usage: mysqldumpslow [ OPTS... ] [ LOGS... ]

Parse and summarize the MySQL slow query log. Options are

 --verbose    verbose
 --debug      debug
 --help       write this text to standard output

 -v           verbose
 -d           debug          //查錯
 -s ORDER     what to sort by (t, at, l, al, r, ar etc), 'at' is default     //排序方式query次數，時間，lock的時間和返回的記錄數來排序
 -r           reverse the sort order (largest last instead of first)       //倒排序
 -t NUM       just show the top n queries                                       //顯示前N多個
 -a           don't abstract all numbers to N and strings to 'S'
 -n NUM       abstract numbers with at least n digits within names   //抽象的數字，至 少有n位內的名稱
 -g PATTERN   grep: only consider stmts that include this string      //配置模式
 -h HOSTNAME  hostname of db server for *-slow.log filename (can be wildcard),     //mysql因此機器名或者IP
 default is '*', i.e. match all
 -i NAME      name of server instance (if using mysql.server startup script)
 -l           don't subtract lock time from total time           //總時間中不減去鎖定時間

例子：

[root@BlackGhost bin]# ./mysqldumpslow -s r -t 20 /usr/local/mysql/mysql-slow.log

[root@BlackGhost bin]# ./mysqldumpslow -s r -t 20 -g 'count' /usr/local/mysql/mysql-slow.log