Fail to alter integer column with a foreign key

[YangKeao]

Bug Report

1. Minimal reproduce step (Required)

mysql> create table parent(id bigint(10) key);
Query OK, 0 rows affected (0.05 sec)

mysql> create table child(pid bigint(20), foreign key (pid) references parent(id));
Query OK, 0 rows affected (0.03 sec)

mysql> alter table child modify column pid bigint(1);
ERROR 1832 (HY000): Cannot change column 'pid': used in a foreign key constraint 'fk_1'

2. What did you expect to see? (Required)

mysql> create table parent(id bigint(10) key);
Query OK, 0 rows affected, 1 warning (0.01 sec)

mysql> create table child(pid bigint(20), foreign key (pid) references parent(id));
Query OK, 0 rows affected, 1 warning (0.02 sec)

mysql> alter table child modify column pid bigint(1);
Query OK, 0 rows affected, 1 warning (0.00 sec)
Records: 0  Duplicates: 0  Warnings: 1

3. What did you see instead (Required)

TiDB failed to alter table.

4. What is your TiDB version? (Required)

master 2482a6e

[YangKeao]

The following lines of checkModifyColumnWithForeignKeyConstraint gives this error:

				if newCol.GetFlen() < referCol.GetFlen() || newCol.GetFlen() < originalCol.GetFlen() ||
					(newCol.GetType() == mysql.TypeNewDecimal && (newCol.GetFlen() != originalCol.GetFlen() || newCol.GetDecimal() != originalCol.GetDecimal())) {
					return dbterror.ErrForeignKeyColumnCannotChange.GenWithStackByArgs(originalCol.Name, fkInfo.Name)
				}

Actually this condition can be relaxed, because the length of integer does nothing on the data 🤔 . See the needChangeColumnData:

		case mysql.TypeTiny, mysql.TypeShort, mysql.TypeInt24, mysql.TypeLong, mysql.TypeLonglong:
			return toUnsigned != originUnsigned

[yoshikipom]

/assign