mysql 获取执行计划方法： 1.通过explain进行查看sql的执行计划； 2.通线程正在执行的sql查看该sql的执行计划；
explain进行查看sql的执行计划相对简单，其实通线程正在执行的sql查看执行计划也很简单，那么为什么还需要第二种方式呢？
因为日常工作会发现很多sql一直在执行，执行发生异常，如果把该sql拿出来执行又很快，于是我们想获取该sql执行异常时候的执行计划，同时因为数据在时刻的变化，统计信息也有可能变化，有可能导致执行计划发生改变，好在mysql5.7开始已经可以通过正在执行的线程查看该sql的执行计划，不需要把sql拿出来执行获取执行计划，这样就可以很好的还原真相：
show processlist;或者select * from information_schema.PROCESSLIST where info is not null order by time desc ;查看。
mysql> select * from information_schema.PROCESSLIST where info is not null;
+-----+------+-----------------+------+---------+------+--------------+-----------------------------------------------------------------------+
| ID | USER | HOST | DB | COMMAND | TIME | STATE | INFO |
+-----+------+-----------------+------+---------+------+--------------+-----------------------------------------------------------------------+
| 17 | root | localhost | NULL | Query | 0 | executing | select * from information_schema.PROCESSLIST where info is not null |
| 178 | root | localhost:60124 | app1 | Query | 560 | Sending data | select count(*) from t ,t_adress where t.adress = t_adress.adress |
+-----+------+-----------------+------+---------+------+--------------+-----------------------------------------------------------------------+
2 rows in set (0.00 sec)
mysql> EXPLAIN FOR CONNECTION 178; +----+-------------+----------+------------+------+---------------+------+---------+------+--------+----------+----------------------------------------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+----------+------------+------+---------------+------+---------+------+--------+----------+----------------------------------------------------+ | 1 | SIMPLE | t_adress | NULL | ALL | NULL | NULL | NULL | NULL | 200096 | 100.00 | NULL | | 1 | SIMPLE | t | NULL | ALL | NULL | NULL | NULL | NULL | 309664 | 100.00 | Using where; Using join buffer (Block Nested Loop) | +----+-------------+----------+------------+------+---------------+------+---------+------+--------+----------+----------------------------------------------------+ 2 rows in set (0.01 sec)
使用起来是不是很方便？
最后说一句： 今天心里很憋屈，睡觉了。。。。。。。。
