请教老师一个问题：老师在课程中提到memory存储引擎的表，是表级锁。我的理解是，如果对表的记录进行DML操作，会带来表级锁。而，我在验证的试试，并不是如我理解的那样：如下：
会话1：

mysql> show variables like '%iso%';
+---------------+-----------------+
| Variable_name | Value           |
+---------------+-----------------+
| tx_isolation  | REPEATABLE-READ |
+---------------+-----------------+
1 row in set (0.00 sec)

mysql> show create table mymemory\G
*************************** 1. row ***************************
       Table: mymemory
Create Table: CREATE TABLE `mymemory` (
  `id` int(11) DEFAULT NULL
) ENGINE=MEMORY DEFAULT CHARSET=latin1
1 row in set (0.00 sec)

mysql> select * from mymemory;
+------+
| id   |
+------+
|   11 |
|    2 |
+------+
2 rows in set (0.00 sec)

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

mysql> update mymemory set id=1 where id=11;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> 

会话2：

mysql> show variables like '%iso%';
+---------------+-----------------+
| Variable_name | Value           |
+---------------+-----------------+
| tx_isolation  | REPEATABLE-READ |
+---------------+-----------------+
1 row in set (0.00 sec)

mysql> select * from mymemory;
+------+
| id   |
+------+
|    1 |
|    2 |
+------+
2 rows in set (0.00 sec)

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

mysql> update mymemory set id=22 where id=2;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

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

mysql> select * from mymemory;
+------+
| id   |
+------+
|    1 |
|   22 |
|    3 |
+------+
3 rows in set (0.00 sec)

mysql> 

从上，按照我的理解，memory存储引擎的表级锁。应该是：会话1，开启事务，去更新id=1的记录，此时会获取该表的表级锁，也就意味着，在该事务未完成(提交或回滚）之前，其它会话，即使去更新该表的其它行记录时，也会被阻塞。可是，却不是我所理解的那样。请老师帮忙解释一下，此处的表级锁的含义，谢谢！