想创建一个列的默认值为sysdate提示语法错误，是不支持这种定义方式么？
MySQL [test]> create table test3(dd timestamp DEFAULT sysdate());
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your TiDB version for the right syntax to use line 1 column 47 near “sysdate())”
MySQL [test]> create table test2(dd timestamp DEFAULT current_timestamp());
Query OK, 0 rows affected (0.54 sec)
