通过status命令，查看Slow queries这一项，如果值长时间>0,说明有查询执行时间过长
mysql> status;
--------------
mysql Ver 11.18 Distrib 3.23.58, for redhat-linux-gnu (i386)
Connection id: 53
Current database: (null)
Current user: root@localhost
Current pager: stdout
Using outfile: ''
Server version: 5.0.37-log
Protocol version: 10
Connection: Localhost via UNIX socket
Client characterset: latin1
Server characterset: latin1
UNIX socket: /tmp/mysql.sock
Uptime: 4 days 16 hours 49 min 57 sec
Threads: 1 Questions: 706 Slow queries: 0 Opens: 177 Flush tables: 1 Open tables:
52 Queries per second avg: 0.002
这时再通过show processlist命令来查看当前正在运行的SQL，从中找出运行慢的SQL语句，找到执行慢的语句后，再用explain命令查看这些语句的执行计划。
mysql> show processlist;
+----+------+-----------+------+---------+------+-------+------------------+
| Id | User | Host | db | Command | Time | State | Info |
+----+------+-----------+------+---------+------+-------+------------------+
| 53 | root | localhost | NULL | Query | 0 | NULL | show processlist |
+----+------+-----------+------+---------+------+-------+------------------+
