问在InnoDB中通过辅助索引删除行时，锁是如何工作的？

EN

CREATE TABLE `follows` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `followable_type` varchar(255) DEFAULT NULL,
  `followable_id` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `type_first` (`followable_type`,`followable_id`),
  KEY `id_first` (`followable_id`,`followable_type`)
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;

insert into follows(followable_type, followable_id) values ("Post", 6);
insert into follows(followable_type, followable_id) values ("Post", 22);
insert into follows(followable_type, followable_id) values ("Post", 28);
insert into follows(followable_type, followable_id) values ("Post", 32);
insert into follows(followable_type, followable_id) values ("Post", 34);

session1> begin;
session1> delete from follows where followable_id=28;

session2> begin;
session2> insert into follows(followable_type, followable_id) value ("Post", 22);  
Lock wait timeout exceeded; try restarting transaction

session2>insert into follows(followable_type, followable_id) value ("Post", 28);  
Lock wait timeout exceeded; try restarting transaction

session2>insert into follows(followable_type, followable_id) value ("Post", 32);  
Query OK, 1 row affected (0.01 sec)