MySQL中的慢查询日志(Slow Query Log)

MySQL中的慢查询日志(Slow Query Log)

1. 以什么形式来记录？
log_output系统变量来决定的，可选值 TABLE, FILE, 或者 NONE，默认值是FILE，可以同时选择TABLE和FILE，中间用逗号隔开。

2. 如何开启？
设置 slow_query_log 系统变量，设置为ON,OFF

3. 具体写到哪里？
如果是写到文件里，由slow_query_log_file 指定，如果要看默认值，可以使用show variables like 'slow_query_log_file'。如果写到表里，则写到mysql.slow_log

4. 何时开启？
可以在启动MySQL server的时候开启，指定--log_output，--slow_query_log和--slow_query_log_file选项；也可以在运行时开启，设置环境变量log_output，slow_query_log和slow_query_log_file

5. 记录什么内容？
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.

6. 多慢算慢？
非administrative 语句，执行时间比long_query_time长的，并且返回的行数不少于min_examined_row_limit的，都算慢

测试：
下面的测试中，设置long_query_time为4s，即如果SQL执行时间长于4s钟，就记录到慢日志中：

mysql> set global slow_query_log=1;
mysql> set global long_query_time=4;
mysql> show variables like 'slow_query_log_file';
+---------------------+------------------------------+
| Variable_name | Value |
+---------------------+------------------------------+
| slow_query_log_file | /var/lib/mysql/db2a-slow.log |
+---------------------+------------------------------+
1 row in set (0.00 sec)

mysql> show variables like 'long_query_time';
+-----------------+----------+
| Variable_name | Value |
+-----------------+----------+
| long_query_time | 4.000000 |
+-----------------+----------+
1 row in set (0.01 sec)

mysql> show variables like 'min_examined_row_limit';
+------------------------+-------+
| Variable_name | Value |
+------------------------+-------+
| min_examined_row_limit | 0 |
+------------------------+-------+
1 row in set (0.00 sec)

mysql> select * from t1 where id < 1000000 order by word desc limit 1;
+--------+--------------------------------------+----------------------------+
| id | word | curtime |
+--------+--------------------------------------+----------------------------+
| 484424 | zZzzAs9yRCb7Jwcdskb95dZR2GSVO85Hbg== | 2017-09-13 22:08:41.277430 |
+--------+--------------------------------------+----------------------------+
1 row in set ( 4.80 sec)

mysql> select * from test1 limit 1;
+------+
| year |
+------+
| 2010 |
+------+
1 row in set ( 0.05 sec)

mysql> create table t2 like t1;
Query OK, 0 rows affected (0.27 sec)

mysql> insert into t2 select * from t1 where id < 100000;
Query OK, 99999 rows affected ( 10.40 sec)
Records: 99999 Duplicates: 0 Warnings: 0

mysql> select * from t2 where id < 1000000 order by word desc limit 1;
+-------+--------------------------------------+----------------------------+
| id | word | curtime |
+-------+--------------------------------------+----------------------------+
| 61356 | zZzqYVLg7cleB2ku281C2TfwAUc51P52mg== | 2017-09-13 20:59:03.136307 |
+-------+--------------------------------------+----------------------------+
1 row in set ( 0.06 sec)

mysql> set global slow_query_log=0;

下面来看下慢日志的内容：

root@db2a:~# cat /var/lib/mysql/db2a-slow.log
mysqld, Version: 5.7.18-log (MySQL Community Server (GPL)). started with:
Tcp port: 3306 Unix socket: /var/run/mysqld/mysqld.sock
Time Id Command Argument
# Time: 2017-09-18T13:04:29.577414Z
# User@Host: root[root] @ localhost [] Id: 16
# Query_time: 4.795787 Lock_time: 0.000232 Rows_sent: 1 Rows_examined: 10000001
use sample;
SET timestamp=1505739869;
select * from t1 where id < 1000000 order by word desc limit 1;
# Time: 2017-09-18T13:05:15.201835Z
# User@Host: root[root] @ localhost [] Id: 16
# Query_time: 10.399761 Lock_time: 0.000464 Rows_sent: 0 Rows_examined: 10000000
SET timestamp=1505739915;
insert into t2 select * from t1 where id < 100000;
mysqld, Version: 5.7.18-log (MySQL Community Server (GPL)). started with:
Tcp port: 3306 Unix socket: /var/run/mysqld/mysqld.sock
Time Id Command Argument

可以看到，有两条符合结果的记录，而且详细记录了Query_time、Lock_time、Rows_examined。

参考链接：
https://dev.mysql.com/doc/refman/5.7/en/log-destinations.html
https://dev.mysql.com/doc/refman/5.7/en/slow-query-log.html