delete ignore foreign key check failed

[crazycs520]

Reproduce

CREATE TABLE t1 (a INT PRIMARY KEY);
CREATE TABLE t2 (b INT);
INSERT INTO t1 VALUES (1), (2), (5);
INSERT INTO t2 VALUES (1), (5);
ALTER TABLE t2 ADD CONSTRAINT FOREIGN KEY(b) REFERENCES t1(a);
DELETE IGNORE FROM t1;

The last DELETE statement should execute success, but failed in TiDB.

> DELETE IGNORE FROM t1;
(1451, 'Cannot delete or update a parent row: a foreign key constraint fails (`test`.`t2`, CONSTRAINT `fk_1` FOREIGN KEY (`b`) REFERENCES `t1` (`a`))')

TiDB Version

master: 2ea253e