MySQL优化之慢查询日志

By | 2020年4月2日

慢查询日志概述

所谓慢查询日志,就是用于记录MySQL中响应时间超过设定阈值的SQL语句,通过打开慢查询开关,MySQL会将大于阈值的SQL记录在日志中,以便于分析性能。

慢查询日志选项默认是关闭的,如果要开启,则需要手动设置。
慢查询日志选项不建议一直开启,因为记录日志意味着IO操作,本身对性能有一定的影响,因此,建议在生产环境关闭该选项;而在开发环境调优阶段可以适当打开该选项。
检查是否开启了慢查询日志:

mysql> show variables like '%slow_query_log%';
+---------------------+-----------------------------------------+
| Variable_name       | Value                                   |
+---------------------+-----------------------------------------+
| slow_query_log      | OFF                                     |
| slow_query_log_file | /var/lib/mysql/DESKTOP-Q5J25HR-slow.log |
+---------------------+-----------------------------------------+
2 rows in set (0.03 sec)

slow_query_log选项即开启慢查询的开关,OFF为关闭,ON为开启。
slow_query_log_file即慢查询日志的名称。

如果要开启慢查询日志,则执行如下语句:

set global slow_query_log = ON;
/* 或者 */
set global slow_query_log = 1;

以上两句话是一个意思,执行任意一条都可以。执行完毕,再次查询,已经打开。

mysql> set global slow_query_log = ON;
Query OK, 0 rows affected (0.05 sec)

mysql> show variables like '%slow_query_log%';
+---------------------+-----------------------------------------+
| Variable_name       | Value                                   |
+---------------------+-----------------------------------------+
| slow_query_log      | ON                                      |
| slow_query_log_file | /var/lib/mysql/DESKTOP-Q5J25HR-slow.log |
+---------------------+-----------------------------------------+
2 rows in set (0.00 sec)

同样的,慢查询日志的文件名也可以重新指定,只需要设置set global slow_query_log_file = 文件名即可。

前面说过,慢查询日志只有当SQL语句响应时间超过一定阈值才会记录在日志中,那么这个阈值是多少,该如何设置呢?
可通过以下命令查看慢查询日志的阈值:

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

可以看到,默认的阈值是10秒,该值同样可以通过设置来修改,如设置为0.1秒:

set global long_query_time = 0.1;

long_query_time设置完成后,需要退出MySQL客户端,重新登录后才生效,于是可以看到该值已经变成了0.1:

mysql> show variables like '%long_query_time%';
+-----------------+----------+
| Variable_name   | Value    |
+-----------------+----------+
| long_query_time | 0.100000 |
+-----------------+----------+
1 row in set (0.01 sec)

慢查询日志具体案例

接下来通过案例的方式来演示慢查询日志:
我在当前数据库里有一张test01表,表结构如下所示:

mysql> desc test01;
+--------+-------------+------+-----+---------+-------+
| Field  | Type        | Null | Key | Default | Extra |
+--------+-------------+------+-----+---------+-------+
| id     | int(4)      | YES  | MUL | NULL    |       |
| name   | varchar(20) | YES  |     | NULL    |       |
| passwd | char(20)    | YES  |     | NULL    |       |
| inf    | char(50)    | YES  |     | NULL    |       |
+--------+-------------+------+-----+---------+-------+
4 rows in set (0.04 sec)

分别做如下查询:

select * from test01;
select sleep(1);
select id, sleep(2) from test01;

执行结果如下所示:

mysql> select * from test01;
+------+------+--------+--------+
| id   | name | passwd | inf    |
+------+------+--------+--------+
|    1 | zz   | 123456 | asdfgh |
+------+------+--------+--------+
1 row in set (0.00 sec)

mysql> select sleep(1);
+----------+
| sleep(1) |
+----------+
|        0 |
+----------+
1 row in set (1.00 sec)

mysql> select id, sleep(2) from test01;
+------+----------+
| id   | sleep(2) |
+------+----------+
|    1 |        0 |
+------+----------+
1 row in set (2.00 sec)

此时并需要去不关心SQL本身,而是看这三条SQL执行的时间,从上面可以看到,第一条SQL执行几乎不耗时间,显示0.00 sec,第二条SQL显示时间为 1.00 sec,第三条SQL为 2.00 sec
因为我们设置了阈值为0.1秒,因此后两条SQL应该都要记录在慢查询日志中,接下来验证是否如此:
通过如下命令,可查看慢查询SQL的条数:

mysql> show global status like '%slow_queries%';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| Slow_queries  | 2     |
+---------------+-------+
1 row in set (0.02 sec)

显示记录为2条,与实际情况一致。
通过show variables like '%slow_query_log%'命令,可以查看到日志的具体路径。

mysql> show variables like '%slow_query_log%';
+---------------------+-----------------------------------------+
| Variable_name       | Value                                   |
+---------------------+-----------------------------------------+
| slow_query_log      | ON                                      |
| slow_query_log_file | /var/lib/mysql/DESKTOP-Q5J25HR-slow.log |
+---------------------+-----------------------------------------+
2 rows in set (0.00 sec)

打开/var/lib/mysql/DESKTOP-Q5J25HR-slow.log,可以看到日志当中记录了时间超过阈值的那两条SQL语句:

/usr/sbin/mysqld, Version: 5.7.29-0ubuntu0.18.04.1 ((Ubuntu)). started with:
Tcp port: 3306  Unix socket: /var/run/mysqld/mysqld.sock
Time                 Id Command    Argument
/usr/sbin/mysqld, Version: 5.7.29-0ubuntu0.18.04.1 ((Ubuntu)). started with:
Tcp port: 3306  Unix socket: /var/run/mysqld/mysqld.sock
Time                 Id Command    Argument
# Time: 2020-03-31T13:41:25.726554Z
# User@Host: root[root] @ localhost []  Id:     6
# Query_time: 1.000644  Lock_time: 0.000000 Rows_sent: 1  Rows_examined: 0
use testDB;
SET timestamp=1585662085;
select sleep(1);
# Time: 2020-03-31T13:41:34.246770Z
# User@Host: root[root] @ localhost []  Id:     6
# Query_time: 2.000536  Lock_time: 0.000057 Rows_sent: 1  Rows_examined: 1
SET timestamp=1585662094;
select id, sleep(2) from test01;

mysqldumpslow工具

以上介绍的方法虽然可以查看出所有慢SQL语句,但显然都记录在一个日志文件里,会显得很乱,而且一旦SQL比较多,定位起来还是比较麻烦的。
好在MySQL本身提供了一个专门用来查看慢查询日志的工具,即mysqldumpslow。
可通过MySQLdumpslow -help查看具体使用方法:

chenyc@DESKTOP-Q5J25HR:~$ 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
                ar: average rows sent
                at: average query time
                 c: count
                 l: lock time
                 r: rows sent
                 t: query time
  -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

从以上文件中,可以知道,该工具用法如下:

mysqldumpslow [选项] [日志名]

常用选项说明:

  • -s 即order排序,后面可以有以下选项:
    • al 平均锁定时间
    • ar 平均返回记录时间
    • at 平均查询时间
    • c 计数
    • l 锁定时间
    • r 逆序排序
    • t 查询时间
  • -t,相当于top n,即返回前面n条语句
  • -g, 匹配正则表达式,大小写不敏感
    如:
sudo mysqldumpslow -s r -t 3 -g 'select' /var/lib/mysql/DESKTOP-Q5J25HR-slow.log

以上命令表示:返回按逆序排序的其中三条语句,且语句中包含select的SQL语句。
结果如下所示:

chenyc@DESKTOP-Q5J25HR:~$ sudo mysqldumpslow -s r -t 3 -g 'select' /var/lib/mysql/DESKTOP-Q5J25HR-slow.log

Reading mysql slow query log from /var/lib/mysql/DESKTOP-Q5J25HR-slow.log
Count: 1  Time=2.00s (2s)  Lock=0.00s (0s)  Rows=1.0 (1), root[root]@localhost
  select id, sleep(N) from test01

Count: 1  Time=1.00s (1s)  Lock=0.00s (0s)  Rows=1.0 (1), root[root]@localhost
  select sleep(N)

Died at /usr/bin/mysqldumpslow line 167, <> chunk 2.

Profiles工具

profiles最大的作用是用来分析海量数据,该命令会记录所有执行过的SQL语句。
这个选项默认也是关闭的,需要手动打开。
查询profiles选项命令:

mysql> show variables like '%profiling%';
+------------------------+-------+
| Variable_name          | Value |
+------------------------+-------+
| have_profiling         | YES   |
| profiling              | OFF   |
| profiling_history_size | 15    |
+------------------------+-------+
3 rows in set (0.01 sec)

打开命令:

mysql> set profiling = ON;
Query OK, 0 rows affected, 1 warning (0.00 sec)

再次查看,可以换一种方式查看:

mysql> select @@profiling;
+-------------+
| @@profiling |
+-------------+
|           1 |
+-------------+
1 row in set, 1 warning (0.00 sec)

以上选项,1代表打开,0代表关闭。
为了更为直观的查看profile,我们再执行两条SQL:

mysql> select sleep(3);
+----------+
| sleep(3) |
+----------+
|        0 |
+----------+
1 row in set (3.00 sec)

mysql> select * from test01;
+------+------+--------+--------+
| id   | name | passwd | inf    |
+------+------+--------+--------+
|    1 | zz   | 123456 | asdfgh |
+------+------+--------+--------+
1 row in set (0.00 sec)

好了,准备工作就到这里,现在来查看profile:

mysql> show profiles;
+----------+------------+-----------------------------------+
| Query_ID | Duration   | Query                             |
+----------+------------+-----------------------------------+
|        1 | 0.00214400 | show variables like '%profiling%' |
|        2 | 0.00017400 | select @@profiling                |
|        3 | 3.00062925 | select sleep(3)                   |
|        4 | 0.00023450 | select * from test01              |
+----------+------------+-----------------------------------+
4 rows in set, 1 warning (0.00 sec)

如上所示,它会记录所有的语句,其中第二列Duration代表的是执行时间。
以上虽然可以看到每条SQL的执行时间,但我们无法精确知道有多少时间花费在IO上,多少时间花费在CPU上,因此,还可使用以下语句查看更为精确的内容:

show profile all for query $(Query_ID);

上面的Query_IDshow profiles结果中第一列的id编号。如:

mysql> show profile all for query 4;
+----------------------+----------+----------+------------+-------------------+---------------------+--------------+---------------+---------------+-------------------+-------------------+-------------------+-------+-----------------------+----------------------+-------------+
| Status               | Duration | CPU_user | CPU_system | Context_voluntary | Context_involuntary | Block_ops_in | Block_ops_out | Messages_sent | Messages_received | Page_faults_major | Page_faults_minor | Swaps | Source_function       | Source_file
    | Source_line |
+----------------------+----------+----------+------------+-------------------+---------------------+--------------+---------------+---------------+-------------------+-------------------+-------------------+-------+-----------------------+----------------------+-------------+
| starting             | 0.000045 | 0.000000 |   0.000000 |                 0 |                   0 |            0 |
0 |             0 |                 0 |                 0 |                 0 |     0 | NULL                  | NULL
    |        NULL |
| checking permissions | 0.000008 | 0.000000 |   0.000000 |                 0 |                   0 |            0 |
0 |             0 |                 0 |                 0 |                 0 |     0 | check_access          | sql_authorization.cc |         809 |
| Opening tables       | 0.000020 | 0.000000 |   0.000000 |                 0 |                   0 |            0 |
0 |             0 |                 0 |                 0 |                 0 |     0 | open_tables           | sql_base.cc
    |        5781 |
| init                 | 0.000017 | 0.000000 |   0.000000 |                 0 |                   0 |            0 |
0 |             0 |                 0 |                 0 |                 0 |     0 | handle_query          | sql_select.cc
    |         128 |
| System lock          | 0.000016 | 0.000000 |   0.000000 |                 0 |                   0 |            0 |
0 |             0 |                 0 |                 0 |                 2 |     0 | mysql_lock_tables     | lock.cc
    |         330 |
| optimizing           | 0.000005 | 0.000000 |   0.000000 |                 0 |                   0 |            0 |
0 |             0 |                 0 |                 0 |                 0 |     0 | optimize              | sql_optimizer.cc
    |         158 |
| statistics           | 0.000014 | 0.000000 |   0.000000 |                 0 |                   0 |            0 |
0 |             0 |                 0 |                 0 |                 0 |     0 | optimize              | sql_optimizer.cc
    |         374 |
| preparing            | 0.000010 | 0.000000 |   0.000000 |                 0 |                   0 |            0 |
0 |             0 |                 0 |                 0 |                 0 |     0 | optimize              | sql_optimizer.cc
    |         482 |
| executing            | 0.000004 | 0.000000 |   0.000000 |                 0 |                   0 |            0 |
0 |             0 |                 0 |                 0 |                 0 |     0 | exec                  | sql_executor.cc
    |         126 |
| Sending data         | 0.000051 | 0.000000 |   0.000000 |                 0 |                   0 |            0 |
0 |             0 |                 0 |                 0 |                 8 |     0 | exec                  | sql_executor.cc
    |         202 |
| end                  | 0.000005 | 0.000000 |   0.000000 |                 0 |                   0 |            0 |
0 |             0 |                 0 |                 0 |                 0 |     0 | handle_query          | sql_select.cc
    |         206 |
| query end            | 0.000007 | 0.000000 |   0.000000 |                 0 |                   0 |            0 |
0 |             0 |                 0 |                 0 |                 0 |     0 | mysql_execute_command | sql_parse.cc
    |        4956 |
| closing tables       | 0.000007 | 0.000000 |   0.000000 |                 0 |                   0 |            0 |
0 |             0 |                 0 |                 0 |                 0 |     0 | mysql_execute_command | sql_parse.cc
    |        5009 |
| freeing items        | 0.000013 | 0.000000 |   0.000000 |                 0 |                   0 |            0 |
0 |             0 |                 0 |                 0 |                 0 |     0 | mysql_parse           | sql_parse.cc
    |        5622 |
| cleaning up          | 0.000013 | 0.000000 |   0.000000 |                 0 |                   0 |            0 |
0 |             0 |                 0 |                 0 |                 0 |     0 | dispatch_command      | sql_parse.cc
    |        1931 |
+----------------------+----------+----------+------------+-------------------+---------------------+--------------+-------------
--+---------------+-------------------+-------------------+-------------------+-------+-----------------------+------------------
----+-------------+
15 rows in set, 1 warning (0.00 sec)

上面的结果由于选项太多,看起来还是有些乱,其实很多东西我们并不关注,而主要只关注CPU和IO情况,因此,可以进一步简化如下:

mysql> show profile cpu, block io for query 4;
+----------------------+----------+----------+------------+--------------+---------------+
| Status               | Duration | CPU_user | CPU_system | Block_ops_in | Block_ops_out |
+----------------------+----------+----------+------------+--------------+---------------+
| starting             | 0.000045 | 0.000000 |   0.000000 |            0 |             0 |
| checking permissions | 0.000008 | 0.000000 |   0.000000 |            0 |             0 |
| Opening tables       | 0.000020 | 0.000000 |   0.000000 |            0 |             0 |
| init                 | 0.000017 | 0.000000 |   0.000000 |            0 |             0 |
| System lock          | 0.000016 | 0.000000 |   0.000000 |            0 |             0 |
| optimizing           | 0.000005 | 0.000000 |   0.000000 |            0 |             0 |
| statistics           | 0.000014 | 0.000000 |   0.000000 |            0 |             0 |
| preparing            | 0.000010 | 0.000000 |   0.000000 |            0 |             0 |
| executing            | 0.000004 | 0.000000 |   0.000000 |            0 |             0 |
| Sending data         | 0.000051 | 0.000000 |   0.000000 |            0 |             0 |
| end                  | 0.000005 | 0.000000 |   0.000000 |            0 |             0 |
| query end            | 0.000007 | 0.000000 |   0.000000 |            0 |             0 |
| closing tables       | 0.000007 | 0.000000 |   0.000000 |            0 |             0 |
| freeing items        | 0.000013 | 0.000000 |   0.000000 |            0 |             0 |
| cleaning up          | 0.000013 | 0.000000 |   0.000000 |            0 |             0 |
+----------------------+----------+----------+------------+--------------+---------------+
15 rows in set, 1 warning (0.00 sec)

全局查询日志

同profiles一样,打开全局查询日志选项以后,它会记录下所有SQL语句,但是由于该操作比较耗费性能,因此,只建议在开发环境临时打开该选项。
查看全局查询日志命令:

mysql> show variables like '%general_log%';
+------------------+------------------------------------+
| Variable_name    | Value                              |
+------------------+------------------------------------+
| general_log      | OFF                                |
| general_log_file | /var/lib/mysql/DESKTOP-Q5J25HR.log |
+------------------+------------------------------------+
2 rows in set (0.01 sec)

打开全局查询日志选项:

set global general_log = on;

再次查询:

mysql> show variables like '%general_log%';
+------------------+------------------------------------+
| Variable_name    | Value                              |
+------------------+------------------------------------+
| general_log      | ON                                 |
| general_log_file | /var/lib/mysql/DESKTOP-Q5J25HR.log |
+------------------+------------------------------------+
2 rows in set (0.00 sec)

设置完以上之后,还需要做如下设置:

set global log_output = 'table';

同样的,准备以下SQL:

select * from test01;
select id, name from test01 where id = 1;

全局查询日志记录在mysql.general_log表中。

mysql> select * from mysql.general_log;
+----------------------------+---------------------------+-----------+-----------+--------------+------------------------------------------+
| event_time                 | user_host                 | thread_id | server_id | command_type | argument
          |
+----------------------------+---------------------------+-----------+-----------+--------------+------------------------------------------+
| 2020-03-31 22:31:24.750895 | root[root] @ localhost [] |         6 |         0 | Query        | select * from test01
          |
| 2020-03-31 22:31:26.128924 | root[root] @ localhost [] |         6 |         0 | Query        | select id, name from test01 whe
re id = 1 |
| 2020-03-31 22:32:15.736558 | root[root] @ localhost [] |         6 |         0 | Query        | select * from mysql.general_log
          |
+----------------------------+---------------------------+-----------+-----------+--------------+--------------------------------
----------+
3 rows in set (0.01 sec)

既然有记录到表中,自然也有记录到文件中,只需要做如下设置:

set global general_log_file = '/tmp/mysql_general.log';		--设置文件路径
set global log_output = 'file';	--设置记录到文件

可通过如下命令查看文件路径:

mysql> show variables like '%general_log%';
+------------------+------------------------+
| Variable_name    | Value                  |
+------------------+------------------------+
| general_log      | ON                     |
| general_log_file | /tmp/mysql_general.log |
+------------------+------------------------+
2 rows in set (0.00 sec)

同样执行以上两条SQL:

select * from test01;
select id, name from test01 where id = 1;

查看文件内容,得到如下结果:

chenyc@DESKTOP-Q5J25HR:~$ sudo cat /tmp/mysql_general.log
/usr/sbin/mysqld, Version: 5.7.29-0ubuntu0.18.04.1 ((Ubuntu)). started with:
Tcp port: 3306  Unix socket: /var/run/mysqld/mysqld.sock
Time                 Id Command    Argument
2020-03-31T14:35:49.793176Z         6 Query     show variables like '%general_log%'
2020-03-31T14:37:20.241374Z         6 Query     select * from test01
2020-03-31T14:37:21.408617Z         6 Query     select id, name from test01 where id = 1

请关注公众号获取更多资料

发表评论