mysql慢查询

mysql慢查询日志对于跟踪有问题的查询非常有用,可以分析出当前程序里有很耗费资源的sql语句,那如何打开mysql的慢查询日志记录呢?

其实打开mysql的慢查询日志很简单,只需要在mysql的配置文件里(windows系统是my.ini,linux系统是my.cnf)的[mysqld]下面加上如下代码:

log-slow-queries=/var/lib/mysql/slowquery.log  
long_query_time=2 
log-queries-not-using-indexes

注:
log-slow-queries 设置把日志写在那里,为空的时候,系统会给慢查询日志赋予主机名,并被附加slow.log. /var/lib/mysql/slowquery.log为日志存放的文件的位置,一般这个目录要有mysql的运行帐号的可写权限,一般都将这个目录设置为mysql的数据存放目录
long_query_time=2中的2表示查询超过两秒才记录.
log-queries-not-using-indexes 就是字面意思,log下来没有使用索引的query
如果设置了参数log-long-format,那么所有没有使用索引的查询也将被记录。在文件my.cnf或my.ini中加入下面这一行可以记录这些查询这是一个有用的日志。它对于性能的影响不大(假设所有查询都很快),并且强调了那些最需要注意的查询(丢失了索引或索引没有得到最佳应用)
# Time: 070927 8:08:52
# User@Host: root[root] @ [192.168.0.20]
# Query_time: 372 Lock_time: 136 Rows_sent: 152 Rows_examined: 263630
select id, name from manager where id in (66,10135);
这是慢查询日志中的一条,用了372秒,锁了136秒,返回152行,一共查了263630行
如果日志内容很多,用眼睛一条一条去看会累死,mysql自带了分析的工具,使用方法如下:
命令行下,进入mysql/bin目录,输入mysqldumpslow –help或–help可以看到这个工具的参数,主要有
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
-s ORDER     what to sort by (t, at, l, al, r, ar etc), ‘at’ is default
-r           reverse the sort order (largest last instead of first)
-t NUM       just show the top n queries
-a           don’t abstract all numbers to N and strings to ‘S’
-n NUM       abstract numbers with at least n digits within names
-g PATTERN   grep: only consider stmts that include this string
-h HOSTNAME hostname of db server for *-slow.log filename (can be wildcard),
default is ‘*’, i.e. match all
-i NAME      name of server instance (if using mysql.server startup scrīpt)
-l           don’t subtract lock time from total time
-s,是order的顺序,说明写的不够详细,俺用下来,包括看了代码,主要有
c,t,l,r和ac,at,al,ar,分别是按照query次数,时间,lock的时间和返回的记录数来排序,前面加了a的时倒叙
-t,是top n的意思,即为返回前面多少条的数据
-g,后边可以写一个正则匹配模式,大小写不敏感的
mysqldumpslow -s c -t 20 host-slow.log
mysqldumpslow -s r -t 20 host-slow.log
上述命令可以看出访问次数最多的20个sql语句和返回记录集最多的20个sql。
mysqldumpslow -t 10 -s t -g “left join” host-slow.log
这个是按照时间返回前10条里面含有左连接的sql语句。

有资料说要看log_queries_not_using_indexes设置:

mysql> show variables like'%using%';  
+-------------------------------+-------+  
| Variable_name                 | Value |  
+-------------------------------+-------+  
| log_queries_not_using_indexes | ON    |   
+-------------------------------+-------+  
1 row in set (0.00 sec) 

mysqldumpslow — Summarize Slow Query Log Files(http://dev.mysql.com/doc/refman/5.1/en/mysqldumpslow.html)
mysqldumpslow
supports the following options:

  • --helpDisplay a help message and exit.
  • -aDo not abstract all numbers to N and strings to 'S'.
  • --debug, -dRun in debug mode.
  • -g patternConsider only queries that match the (grep-style) pattern.
  • -h host_nameHost name of MySQL server for *-slow.log file name. The value can contain a wildcare. The default is * (match all).
  • -i nameName of server instance (if using mysql.server startup script).
  • -lDo not subtract lock time from total time.
  • -n NAbstract numbers with at least N digits within names.
  • -rReverse the sort order.
  • -s sort_typeHow to sort the output. The value of sort_type should be chosen from the following list:
    • t, at: Sort by query time or average query time
    • l, al: Sort by lock time or average lock time
    • s, as: Sort by rows sent or average rows sent
    • c: Sort by count
  • -t NDisplay only the first N queries in the output.
  • --verbose, -vVerbose mode. Print more information about what the program does.

Example of usage:

shell> mysqldumpslow
Reading mysql slow query log from /usr/local/mysql/data/mysqld51-apple-slow.log
Count: 1  Time=4.32s (4s)  Lock=0.00s (0s)  Rows=0.0 (0), root[root]@localhost
 insert into t2 select * from t1
Count: 3  Time=2.53s (7s)  Lock=0.00s (0s)  Rows=0.0 (0), root[root]@localhost
 insert into t2 select * from t1 limit N
Count: 3  Time=2.13s (6s)  Lock=0.00s (0s)  Rows=0.0 (0), root[root]@localhost
 insert into t1 select * from t1

发表评论