发现了mysql5.6版本之后timestamp的一个有趣问题，具体如下：

mysql> desc test_idx;
+--------+--------------+------+-----+---------+----------------+
| Field  | Type         | Null | Key | Default | Extra          |
+--------+--------------+------+-----+---------+----------------+
| id     | int(11)      | NO   | PRI | NULL    | auto_increment |
| name   | varchar(255) | NO   |     | NULL    |                |
| sid    | int(11)      | NO   | UNI | NULL    |                |
| time_1 | timestamp(6) | YES  |     | NULL    |                |
| time_2 | timestamp(2) | YES  |     | NULL    |                |
| time_3 | timestamp(4) | YES  |     | NULL    |                |
| time_4 | timestamp    | YES  |     | NULL    |                |
+--------+--------------+------+-----+---------+----------------+
7 rows in set (0.00 sec)

mysql> select * from test_idx;
+----+------+-----+----------------------------+------------------------+--------------------------+---------------------+
| id | name | sid | time_1                     | time_2                 | time_3                   | time_4              |
+----+------+-----+----------------------------+------------------------+--------------------------+---------------------+
|  2 | zhao |   1 | 2018-09-20 20:39:59.000000 | 2018-09-20 20:40:05.00 | 2018-09-20 20:40:09.0000 | 2018-09-20 20:40:55 |
+----+------+-----+----------------------------+------------------------+--------------------------+---------------------+
1 row in set (0.00 sec)

问题描述：当建表的时候指定了timestamp的长度(最大为6)的时候，timestamp是可以保持小数后6位的，这6位小数按照官方文档描述是微秒单位。

疑问：是不是说5.6之后timestamp是精确到微秒的，再有timestamp占4个字节，那么底层是怎么存储这6位小数的呢？