我现在正在学习 InnoDB mvcc，我尝试了一个测试节目，如下所示：
mysql版本：
[root@mysql-test ~]# mysql --version
mysql Ver 15.1 Distrib 5.5.52-MariaDB, for Linux (x86_64) using readline 5.1
表架构：
MariaDB [liruifeng]> show create table test_a;
+--------+-------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table |
+--------+-------------------------------------------------------------------------------------------------------------------------------------------------------+
| test_a | CREATE TABLE `test_a` (
`id` int(11) NOT NULL DEFAULT '0',
`a` char(10) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 |
+--------+-------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
然后用这样的数据初始化：
MariaDB [liruifeng]> select * from test_a;
+----+------+
| id | a |
+----+------+
| 1 | 1 |
| 2 | 2 |
| 3 | 3 |
+----+------+
3 rows in set (0.00 sec)
起初我在不同的终端打开了两个会话，测试步骤如下所示：
t1：
MariaDB [liruifeng]> start transaction;
Query OK, 0 rows affected (0.00 sec)
MariaDB [liruifeng]> select * from test_a;
+----+------+
| id | a |
+----+------+
| 1 | 1 |
| 2 | 2 |
| 3 | 3 |
+----+------+
3 rows in set (0.00 sec)
t2：
MariaDB [liruifeng]> insert into test_a values (4,4);
Query OK, 1 row affected (0.01 sec)
MariaDB [liruifeng]> select * from test_a;
+----+------+
| id | a |
+----+------+
| 1 | 1 |
| 2 | 2 |
| 3 | 3 |
| 4 | 4 |
+----+------+
4 rows in set (0.00 sec)
t1：
MariaDB [liruifeng]> select * from test_a;
+----+------+
| id | a |
+----+------+
| 1 | 1 |
| 2 | 2 |
| 3 | 3 |
+----+------+
3 rows in set (0.00 sec)
MariaDB [liruifeng]> update test_a set a = 444 where id = 4;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
MariaDB [liruifeng]> select * from test_a;
+----+------+
| id | a |
+----+------+
| 1 | 1 |
| 2 | 2 |
| 3 | 3 |
| 4 | 444 |
+----+------+
4 rows in set (0.00 sec)
这让我感到困惑，为什么 t1 可以在 t1 提交之前更新 t2 插入的行？我的 tx_isolation 级别是可重复读取的，为什么此更新 sql 有效？
我的隔离显示如下：
MariaDB [liruifeng]> show variables like 'tx_isolation';
+---------------+-----------------+
| Variable_name | Value |
+---------------+-----------------+
| tx_isolation | REPEATABLE-READ |
+---------------+-----------------+
1 row in set (0.00 sec)
感谢提前:)