目的
为了理解MySQL在执行大SQL时，对执行CTRL+C产生的疑惑，本文通过实验测试和源码分析两个方面，对MySQL处理CTRL+C的详细过程进行分析和讲解，从而解除DBA及开发人员对CTRL+C的误解。
测试
首先，基于线上数据库版本，分别使用MySQL客户端版本5.5.20和5.0.77进行实验测试，一方面排除不同数据库客户端版本造成的差异，另一方面，深入了解不同版本执行CTRL+C产生的差异。
MySQL客户端5.5.20
使用MySQL客户端5.5.20在Session1中执行select sleep(100)语句，在Session2中执行show processlist语句；然后在Session1中执行CTRL+C，在Session中执行show processlist语句，查看当前连接的线程。执行的图如下所示：
Session1：
mysql> select sleep(100);
Ctrl-C -- sending "KILL QUERY 153779" to server ...
Ctrl-C -- query aborted.
ERROR 2013 (HY000): Lost connection to MySQL server during query
Session2:
mysql> show processlist;
+--------+-------------+-----------------+------+-------------+---------+-----------------------------------------------------------------------------+-------------------+-----------+---------------+-----------+
| Id | User | Host | db | Command | Time | State | Info | Rows_sent | Rows_examined | Rows_read |
+--------+-------------+-----------------+------+-------------+---------+-----------------------------------------------------------------------------+-------------------+-----------+---------------+-----------+
| 153779 | heng.wang | 127.0.0.1:39882 | NULL | Query | 8 | User sleep | select sleep(100) | 0 | 0 | 1 |
| 153780 | heng.wang | 127.0.0.1:39883 | NULL | Query | 0 | NULL | show processlist | 0 | 0 | 1 |
+--------+-------------+-----------------+------+-------------+---------+-----------------------------------------------------------------------------+-------------------+-----------+---------------+-----------+
2 rows in set (0.00 sec)
mysql> show processlist;
+--------+-------------+-----------------+------+-------------+---------+-----------------------------------------------------------------------------+------------------+-----------+---------------+-----------+
| Id | User | Host | db | Command | Time | State | Info | Rows_sent | Rows_examined | Rows_read |
+--------+-------------+-----------------+------+-------------+---------+-----------------------------------------------------------------------------+------------------+-----------+---------------+-----------+
| 153780 | heng.wang | 127.0.0.1:39883 | NULL | Query | 0 | NULL | show processlist | 0 | 0 | 1 |
+--------+-------------+-----------------+------+-------------+---------+-----------------------------------------------------------------------------+------------------+-----------+---------------+-----------+
1 rows in set (0.00 sec)
从以上结果来看，Session1中执行select操作时，Session2中可以查看该连接正在执行，在Session1中执行CTRL+C时，客户端向服务器端发送KILL QUERY 命令，并且连接关闭。在Session2中可以看到执行select的连接已经关闭。
MySQL客户端5.0.77
使用客户端5.0.77执行同样的操作，执行CTRL+C后，观察执行的差异性。具体如下表中所示：
Session1：
mysql> select sleep(100);
Query aborted by Ctrl+C
+------------+
| sleep(100) |
+------------+
| 1 |
+------------+
Session2：
mysql> show processlist;
+--------+-------------+-----------------+------+-------------+---------+-----------------------------------------------------------------------------+-------------------+-----------+---------------+-----------+
| Id | User | Host | db | Command | Time | State | Info | Rows_sent | Rows_examined | Rows_read |
+--------+-------------+-----------------+------+-------------+---------+-----------------------------------------------------------------------------+-------------------+-----------+---------------+-----------+
| 153783 | heng.wang | 127.0.0.1:45807 | NULL | Query | 3 | User sleep | select sleep(100) | 0 | 0 | 1 |
| 153784 | heng.wang | 127.0.0.1:45809 | NULL | Query | 0 | NULL | show processlist | 0 | 0 | 1 |
+--------+-------------+-----------------+------+-------------+---------+-----------------------------------------------------------------------------+-------------------+-----------+---------------+-----------+
2 rows in set (0.00 sec)
mysql> show processlist;
+--------+-------------+-----------------+------+-------------+---------+-----------------------------------------------------------------------------+------------------+-----------+---------------+-----------+
| Id | User | Host | db | Command | Time | State | Info | Rows_sent | Rows_examined | Rows_read |
+--------+-------------+-----------------+------+-------------+---------+-----------------------------------------------------------------------------+------------------+-----------+---------------+-----------+
| 153783 | heng.wang | 127.0.0.1:45807 | NULL | Sleep | 10 | | NULL | 1 | 0 | 1 |
| 153784 | heng.wang | 127.0.0.1:45809 | NULL | Query | 0 | NULL | show processlist | 0 | 0 | 1 |
+--------+-------------+-----------------+------+-------------+---------+-----------------------------------------------------------------------------+------------------+-----------+---------------+-----------+
2 rows in set (0.00 sec)
从以上结果可知，Session1上执行select时，Session2中建立查询连接；在Session1中执行CTRL+C时，显示Query被终止，并且返回执行的错误结果。在Session2中可知，连接的线程仍然存在，但是Query被终止，只是保持连接。
源码分析
为了更进一步对以上测试进行确认，查看MySQL源码进行进一步的求证，同样基于MySQL的5.5.20和5.0.77两个版本进行验证。具体如下：
MySQL 5.5.20源码
MySQL客户端主函数main中，信号函数在源码文件client/mysql.cc:1163，源码如下所示：
signal(SIGINT, handle_sigint); // Catch SIGINT to clean up
信号处理函数handle_sigint在client/mysql.cc:1291，源码如下所示：
sig_handler handle_sigint(int sig)
{
char kill_buffer[40];
MYSQL *kill_mysql= NULL;
if (!executing_query || (interrupted_query == 2))
{
tee_fprintf(stdout, "Ctrl-C -- exit!\n");
goto err;
}
kill_mysql= mysql_init(kill_mysql);
if (!mysql_real_connect(kill_mysql,current_host, current_user, opt_password,
"", opt_mysql_port, opt_mysql_unix_port,0))
{
tee_fprintf(stdout, "Ctrl-C -- sorry, cannot connect to server to kill query, giving up ...\n");
goto err;
}
interrupted_query++;
if ((interrupted_query == 1) && (mysql_get_server_version(&mysql) < 50000))
interrupted_query= 2;
sprintf(kill_buffer, "KILL %s%lu",
(interrupted_query == 1) ? "QUERY " : "",
mysql_thread_id(&mysql));
tee_fprintf(stdout, "Ctrl-C -- sending \"%s\" to server ...\n", kill_buffer);
mysql_real_query(kill_mysql, kill_buffer, (uint) strlen(kill_buffer));
mysql_close(kill_mysql);
tee_fprintf(stdout, "Ctrl-C -- query aborted.\n");
return;
err:
#ifdef _WIN32
mysql_thread_end();
return;
#else
mysql_end(sig);
#endif
}
从以上源码可知，MySQL客户端在捕获信号后，将KILL QUERY 命令发送到服务器端执行，从而将Query处理KILL。
MySQL 5.0.77源码
MySQL客户端主函数main中，信号函数在源码文件client/mysql.cc:1150。源码如下所示：
signal(SIGINT, mysql_sigint); // Catch SIGINT to clean up
信号处理函数mysql_sigint在源码文件client/mysql.cc:1214，源码如下所示：
sig_handler mysql_sigint(int sig)
{
char kill_buffer[40];
MYSQL *kill_mysql= NULL;
if (!executing_query || interrupted_query++)
goto err;
kill_mysql= mysql_init(kill_mysql);
if (!mysql_real_connect(kill_mysql,current_host, current_user, opt_password,
"", opt_mysql_port, opt_mysql_unix_port,0))
goto err;
sprintf(kill_buffer, "KILL %lu", mysql_thread_id(&mysql));
mysql_real_query(kill_mysql, kill_buffer, strlen(kill_buffer));
mysql_close(kill_mysql);
tee_fprintf(stdout, "Query aborted by Ctrl+C\n");
return;
err:
#ifdef _WIN32
mysql_thread_end();
return;
#else
mysql_end(sig);
#endif
}
通过以上源码可知，MySQL客户端捕获信号后，向服务器端发送KILL 命令并执行，从而将Query处理kill。
基于以上两个版本处理的源码可知，MySQL客户端一定会捕获CTRL+C信号，并对该信号进行处理。而对于不同版本的客户端，由于发送命令的不同，导致MySQL服务器端执行结果有所不同。
结论
通过以上测试和源码分析可知，MySQL客户端肯定会捕获CTRL+C信号，并对信号进行处理。不同的是，在MySQL 5.5.20版本的客户端中执行时，服务器端执行KILL QUERY 命令，将QUERY KILL掉，并将连接关闭。而对MySQL 5.0.77客户端中执行时，服务器端执行KILL 命令，KILL掉QUERY，但保持连接。
此外，对于update、delete数据更新操作[1]，CTRL+C会将执行的操作标记为KILLED状态，然后执行回滚操作。因此，不会因为CTRL+C操作，导致数据变脏。