mysql 更新唯一主键列 被堵塞

mysql> select @@tx_isolation;
+-----------------+
| @@tx_isolation  |
+-----------------+
| REPEATABLE-READ |
+-----------------+
1 row in set (0.00 sec)

Session 196:

mysql> show create table mail;
+-------
| mail  | CREATE TABLE `mail` (
  `mailid` bigint(20) NOT NULL AUTO_INCREMENT,
  `receiverid` int(10) unsigned NOT NULL,
  `senderid` int(10) unsigned NOT NULL,
  PRIMARY KEY (`mailid`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 |

mysql> insert into mail values(1,2222,3333);
Query OK, 1 row affected (0.00 sec)

mysql> insert into mail values(2,333,3333);
Query OK, 1 row affected (0.00 sec)

mysql> commit;
Query OK, 0 rows affected (0.00 sec)

mysql> start transaction;
Query OK, 0 rows affected (0.00 sec)

mysql> insert into mail values(3,333,3333);
Query OK, 1 row affected (0.00 sec)

mysql> select connection_id();
+-----------------+
| connection_id() |
+-----------------+
|             196 |
+-----------------+
1 row in set (0.02 sec)

Session 197:

mysql> update mail set receiverid=193719371 where mailid=3;
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> update mail set receiverid=193719371 where mailid=3;
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> select connection_id();
+-----------------+
| connection_id() |
+-----------------+
|             197 |
+-----------------+
1 row in set (0.00 sec)

mysql> update mail set receiverid=193719371 where mailid=3;

389253799,197,update mail set receiverid=193719371 where mailid=3,389253798,196,
 mysql[192.168.11.187]   blocking_thread[196] current_query[]  blocking waiting_thread[197]'s update mail set receiverid=193719371 where mailid=3
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