To support drop column with index

[blacktear23]

Please answer these questions before submitting your issue. Thanks!

1. What did you do?

create table `t` (`id` int(11), `name` varchar(255), key `idx_name` (`name`));
alter table `t` drop column `name`

2. What did you expect to see?

name column should be deleted with out any error. In MySQL:

mysql> alter table t drop column name;
Query OK, 0 rows affected (0.06 sec)
Records: 0  Duplicates: 0  Warnings: 0

3. What did you see instead?

mysql> alter table t drop column name;
ERROR 1105 (HY000): can't drop column name with index covered now

this "feature" will break some ORM's migration scripts.

4. What version of TiDB are you using (tidb-server -V)?

[breezewish]

Currently we does not support dropping column with index. Please refer to docs about MySQL compatibility.

[blacktear23]

Sure I know that. But the important thing is this will break some ORM's migration script. Such as django-orm.

[breezewish]

Got that. I modified your issue title to better fit your idea.

[zimulala]

It's duplicated with #3364