一，爲什麼要開啓這個查詢呢？
數據庫是很容易產生瓶頸的地方，現在Nosql大家討論這麼熱，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非常慢的語句，這些慢的語句，可能是寫的不夠合理或者是大數據下多表的聯合查詢等等，所以我們要找出這些語句，分析原因，加以優化。這也是發這篇博文的原因
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%"; //查看一下慢查詢是不是已經開啓
+---------------------+---------------------------------+
| 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，不然會報錯的。
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]裏面加上以下內容
1.long_query_time = 2
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。其實自己寫一個shell腳本也是可以把要的信息取出來的。我們來看一下mysql-slow.log裏面到底是什麼東西
[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語句的執行情況，包括執行時間，鎖定時間等，所以要不要分析工具看個人情況，分析工具很多，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所以機器名或者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
