altering a column comment does a full copy

Altering column's comment does a full table copy. It should be executed by only changing the frm.

initial issue description follow

This is a new feature request and is similar to this request. http://bugs.mysql.com/bug.php?id=64439

We have multiple mariadb instances managing ~2.5T of our data. We decided to document them using table/column comments. Unfortunately, there isnt an easier way of doing this.

1) Filed a bug regarding altering table comment doing full copy. MDEV-9103
2) Changing the column comment does the full copy (MDEV-9168)
3) Also, current syntax is error prone since you need to re-declare the data type (MDEV-9878)

It would be nice to have the below:
ALTER TABLE 'myTable' ALTER COLUMN `myField` ADD COMMENT 'a comment'

Instead of:
ALTER TABLE `myTable` CHANGE `myField` `myField` varchar(20) NOT NULL COMMENT 'my comment'

I agree that comments work great, if we have them created while creating the table. But, its almost impossible to update/add them at the later stage for the bigger tables in the production.

Is there a better way of documenting the tables instead of using comments ? Please let me know.

Thanks.

-Bala