bug: alter column comment maybe cause column type change.

[TCeason]

Summary

root@localhost:8000/default/default> CREATE TABLE students_info (
  id INT,
  name VARCHAR(50),
  age INT
);


root@localhost:8000/default/default> ALTER TABLE students_info MODIFY COLUMN age datetime COMMENT 'abc';


root@localhost:8000/default/default> show create table students_info;


*************************** 1. row ***************************
       Table: students_info
Create Table: CREATE TABLE students_info (
  id INT NULL,
  name VARCHAR NULL,
  age TIMESTAMP NULL COMMENT 'abc'
) ENGINE=FUSE