https://www.cnblogs.com/kerrycode/p/5593204.html

慢查询日志概念

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。

官方文档，关于慢查询的日志介绍如下（部分资料，具体参考官方相关链接）：

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

MySQL 慢查询的相关参数解释：

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。

慢查询日志配置

默认情况下slow_query_log的值为OFF，表示慢查询日志是禁用的，可以通过设置slow_query_log的值来开启，如下所示：

like '%slow_query_log%';

| Variable_name       | Value                                         |

| slow_query_log      | OFF                                           |

+---------------------+-----------------------------------------------+

in set (0.00 sec)

mysql> set global slow_query_log=1;

like '%slow_query_log%';

| Variable_name       | Value                                         |

| slow_query_log      | ON                                            |

+---------------------+-----------------------------------------------+

in set (0.00 sec)

mysql> 

like 'slow_query%';

| Variable_name       | Value                                         |

| slow_query_log      | OFF                                           |

+---------------------+-----------------------------------------------+

in set (0.01 sec)

mysql> 

like 'slow_query%';

| Variable_name       | Value               |

| slow_query_log      | ON                  |

+---------------------+---------------------+

in set (0.00 sec)

mysql> 

like 'slow_query_log_file';

| Variable_name       | Value                                         |

| slow_query_log_file | /home/WDPM/MysqlData/mysql/DB-Server-slow.log |

1 row in set (0.00 sec)

like 'long_query_time%';

| Variable_name   | Value     |

| long_query_time | 10.000000 |

1 row in set (0.00 sec)

mysql> set global long_query_time=4;

like 'long_query_time';

| Variable_name   | Value     |

| long_query_time | 10.000000 |

1 row in set (0.00 sec)

select sleep(3);

| sleep(3) |

|        0 |

1 row in set (3.00 sec)

[root@DB-Server ~]# more /tmp/mysql_slow.log

Tcp port: 0  Unix socket: (null)

/usr/sbin/mysqld, Version: 5.6.20-enterprise-commercial-advanced-log (MySQL Enterprise Server - Advanced Edition (Commercial)). started with:

null)

# Time: 160616 17:24:35

# Query_time: 3.002615  Lock_time: 0.000000 Rows_sent: 1  Rows_examined: 0

timestamp=1466069075;

select sleep(3);

like '%log_output%';

| Variable_name | Value |

| log_output    | FILE  |

1 row in set (0.00 sec)

mysql> set global log_output='TABLE';

like '%log_output%';

| Variable_name | Value |

| log_output    | TABLE |

1 row in set (0.00 sec)

mysql> select sleep(5) ;

| sleep(5) |

|        0 |

1 row in set (5.00 sec)

mysql> 

mysql> select * from mysql.slow_log;

| start_time          | user_host                 | query_time | lock_time | rows_sent | rows_examined | db | last_insert_id | insert_id | server_id | sql_text        | thread_id |

| 2016-06-16 17:37:53 | root[root] @ localhost [] | 00:00:03   | 00:00:00  |         1 |             0 |    |              0 |         0 |         1 | select sleep(3) |         5 |

select sleep(5) |         2 |

2 rows in set (0.00 sec)

mysql> 

like 'log_queries_not_using_indexes';

| Variable_name                 | Value |

| log_queries_not_using_indexes | OFF   |

1 row in set (0.00 sec)

mysql> set global log_queries_not_using_indexes=1;

like 'log_queries_not_using_indexes';

| Variable_name                 | Value |

| log_queries_not_using_indexes | ON    |

1 row in set (0.00 sec)

mysql> 

like 'log_slow_admin_statements';

| Variable_name             | Value |

| log_slow_admin_statements | OFF   |

1 row in set (0.00 sec)

mysql> 

global status like '%Slow_queries%';

| Variable_name | Value |

| Slow_queries  | 2104  |

1 row in set (0.00 sec)

mysql> 

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

Parse and summarize the MySQL slow query log. Options are

  --verbose    verbose

  --help       write this text to standard output

  -v           verbose

  -s ORDER     what to sort by (al, at, ar, c, l, r, t), 'at' is default

                ar: average rows sent

                 c: count

                 r: rows sent

  -r           reverse the sort order (largest last instead of first)

  -a           don't abstract all numbers to N and strings to 'S'

  -g PATTERN   grep: only consider stmts that include this string

               default is '*', i.e. match all

  -l           don't subtract lock time from total time