问题背景: mysql server是5.7.20版本，在mysql客户端修改long_query_time全局变量由0.01修改为1但slow log日志仍显示小于1的记录，难道set global long_query_time不起作用吗?
测试一下

root@localhost:mysql3306.sock [(none)]> show global variables like 'long_query_time';
+-----------------+----------+
| Variable_name   | Value    |
+-----------------+----------+
| long_query_time | 1.000000 |
+-----------------+----------+
1 row in set (0.00 sec)

root@localhost:mysql3306.sock [(none)]> show variables like 'long_query_time';
+-----------------+----------+
| Variable_name   | Value    |
+-----------------+----------+
| long_query_time | 1.000000 |
+-----------------+----------+
1 row in set (0.00 sec)

root@localhost:mysql3306.sock [(none)]> set global long_query_time=2;
Query OK, 0 rows affected (0.00 sec)

root@localhost:mysql3306.sock [(none)]> show global variables like 'long_query_time';
+-----------------+----------+
| Variable_name   | Value    |
+-----------------+----------+
| long_query_time | 2.000000 |
+-----------------+----------+
1 row in set (0.00 sec)

root@localhost:mysql3306.sock [(none)]> show variables like 'long_query_time';
+-----------------+----------+
| Variable_name   | Value    |
+-----------------+----------+
| long_query_time | 1.000000 |
+-----------------+----------+
1 row in set (0.01 sec)

结果显示修改long_query_time全局变量后仅全局变量变更了，但当前session的long_query_time没有变更，那原因又是什么呢？
经查官方文档https://dev.mysql.com/doc/refman/5.7/en/server-system-variables.html显示，long_query_time属于全局和会话范围的系统变量

https://dev.mysql.com/doc/refman/8.0/en/using-system-variables.html显示修改全局变量仅对新会话生效而对当前会话不起作用

再查看slow log发现修改全局变量后所谓的不生效日志记录都是同一个Id: 1041729，也就是说这个会话在修改全局变量前就已创建了，这并非是MySQL的Bug~~

如果想立即生效，那该怎么办呢? 一种办法是重启mysqld服务，另一种是kill掉Sleep和Query的连接(mysql -S /tmp/mysql3306.sock -uroot -p -e 'select concat("kill ",thd_id,";") from sys.session where command="Sleep" or command="Query";'|grep kill)；值得注意的是要评估此类操作对业务影响再执行！