介绍

MySQL的慢查询日志是MySQL提供的一种日志记录，它用来记录在MySQL中响应时间超过阀值的语句，具体指运行时间超过long_query_time值的SQL，则会被记录到慢查询日志中。long_query_time的默认值为10，意思是运行10S以上的语句。默认情况下，Mysql数据库并不启动慢查询日志，需要我们手动来设置这个参数，当然，如果不是调优需要的话，一般不建议启动该参数，因为开启慢查询日志会或多或少带来一定的性能影响。慢查询日志支持将日志记录写入文件，也支持将日志记录写入数据库表。

The slow query log consists of SQL statements that took more than long_query_time seconds to execute and required at least min_examined_row_limit rows to be examined. The minimum and default values of long_query_time are 0 and 10, respectively. The value can be specified to a resolution of microseconds. For logging to a file, times are written including the microseconds part. For logging to tables, only integer times are written; the microseconds part is ignored.

By default, administrative statements are not logged, nor are queries that do not use indexes for lookups. This behavior can be changed usinglog_slow_admin_statements and log_queries_not_using_indexes, as described later.

慢查询日志相关参数

属性	说明
slow_query_log	是否开启慢查询日志，1表示开启，0表示关闭。
log-slow-queries	旧版（5.6以下版本）MySQL数据库慢查询日志存储路径。可以不设置该参数，系统则会默认给一个缺省的文件host_name-slow.log
slow-query-log-file	新版（5.6及以上版本）MySQL数据库慢查询日志存储路径。可以不设置该参数，系统则会默认给一个缺省的文件host_name-slow.log
long_query_time	慢查询阈值，当查询时间多于设定的阈值时，记录日志
log_queries_not_using_indexes	未使用索引的查询也被记录到慢查询日志中（可选项）
log_output	日志存储方式。 log_output='FILE'表示将日志存入文件，默认值是'FILE'。 log_output='TABLE'表示将日志存入数据库，这样日志信息就会被写入到mysql.slow_log表中。 MySQL数据库支持同时两种日志存储方式，配置的时候以逗号隔开即可，如：log_output='FILE,TABLE'。日志记录到系统的专用日志表中，要比记录到文件耗费更多的系统资源，因此对于需要启用慢查询日志，又需要能够获得更高的系统性能，那么建议优先记录到文件。

慢查询状态日志管理

查询-慢查询状态

mysql> show variables like '%slow%';
+---------------------------+----------------+
| Variable_name             | Value          |
+---------------------------+----------------+
| log_slow_admin_statements | OFF            |
| log_slow_slave_statements | OFF            |
| slow_launch_time          | 2              |
| slow_query_log            | ON             |
| slow_query_log_file       | SUNLD-slow.log |
+---------------------------+----------------+
5 rows in set, 1 warning (0.03 sec)

查询-查询阈值时间（大于该时间就会记录）

mysql> show variables like '%long_query_time%';
+-----------------+-----------+
| Variable_name   | Value     |
+-----------------+-----------+
| long_query_time | 10.000000 |
+-----------------+-----------+
1 row in set, 1 warning (0.00 sec)

从MySQL 5.1开始，long_query_time开始以微秒记录SQL语句运行时间，之前仅用秒为单位记录。如果记录到表里面，只会记录整数部分，不会记录微秒部分。

注意：使用命令 set global long_query_time=4修改后，需要重新连接或新开一个会话才能看到修改值。你用show variables like 'long_query_time'查看是当前会话的变量值，你也可以不用重新连接会话，而是用show global variables like 'long_query_time';

关闭/启动慢查询服务（重启后失效，永久生效需要修改my.cnf文件）

set global slow_query_log=0; 
set global slow_query_log=1;   

#修改配置文件my.cnf
slow_query_log =1 
slow_query_log_file=/var/lib/mysql/mysql-slow-query-new.log

设置慢查询日志文件路径

set global slow_query_log_file='/var/lib/mysql/mysql-slow-query-new.log';

日志记录方式

mysql> show variables like '%log_output%';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| log_output    | FILE  |
+---------------+-------+
1 row in set, 1 warning (0.00 sec)
#设置存储方式
set global log_output='TABLE';

log_queries_not_using_indexes

系统变量log-queries-not-using-indexes：未使用索引的查询也被记录到慢查询日志中（可选项）。如果调优的话，建议开启这个选项。另外，开启了这个参数，其实使用full index scan的sql也会被记录到慢查询日志。

This option does not necessarily mean that no index is used. For example, a query that uses a full index scan uses an index but would be logged because the index would not limit the number of rows.

mysql> show variables like '%log_queries_not_%';
+-------------------------------+-------+
| Variable_name                 | Value |
+-------------------------------+-------+
| log_queries_not_using_indexes | OFF   |
+-------------------------------+-------+
1 row in set, 1 warning (0.01 sec)
#开启
set global log_queries_not_using_indexes=1;

log_slow_admin_statements

系统变量log_slow_admin_statements表示是否将慢管理语句例如ANALYZE TABLE和ALTER TABLE等记入慢查询日志。

mysql> show variables like '%log_slow_admin%';
+---------------------------+-------+
| Variable_name             | Value |
+---------------------------+-------+
| log_slow_admin_statements | OFF   |
+---------------------------+-------+
1 row in set, 1 warning (0.02 sec)

log_slow_slave_statements

By default, a replication slave does not write replicated queries to the slow query log. To change this, use the log_slow_slave_statements system variable.

When the slow query log is enabled, this variable enables logging for queries that have taken more than long_query_time seconds to execute on the slave. This variable was added in MySQL 5.7.1. Setting this variable has no immediate effect. The state of the variable applies on all subsequent START SLAVE statements.

mysql> show variables like '%log_slow_slave%';
+---------------------------+-------+
| Variable_name             | Value |
+---------------------------+-------+
| log_slow_slave_statements | OFF   |
+---------------------------+-------+
1 row in set, 1 warning (0.02 sec)

参数--log-short-format

The server writes less information to the slow query log if you use the --log-short-format option.

Command-Line Format	--log-short-format
Permitted Values	Type	boolean
	Default	FALSE

查询有多少条慢查询记录

mysql> show global status like '%slow_queries%';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| Slow_queries  | 0     |
+---------------+-------+
1 row in set (0.01 sec)

日志分析工具mysqldumpslow

帮助信息

mysqldumpslow --help
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 (al, at, ar, c, l, r, t), 'at' is default
                al: average lock time 平均锁定时间
                ar: average rows sent 平均返回记录
                at: average query time 平均查询时间
                 c: count 访问次数
                 l: lock time 锁定时间
                 r: rows sent 返回记录
                 t: query time  查询时间
  -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 script)
  -l           don't subtract lock time from total time

#得到返回记录集最多的10个SQL。 
mysqldumpslow -s r -t 10 /database/mysql/mysql06_slow.log 
#得到访问次数最多的10个SQL 
mysqldumpslow -s c -t 10 /database/mysql/mysql06_slow.log 
#得到按照时间排序的前10条里面含有左连接的查询语句。 
mysqldumpslow -s t -t 10 -g “left join” /database/mysql/mysql06_slow.log 
#另外建议在使用这些命令时结合 | 和more 使用 ，否则有可能出现刷屏的情况。 
mysqldumpslow -s r -t 20 /mysqldata/mysql/mysql06-slow.log | more

mysql-慢查询日志slow log

介绍