在MySQL中读取提交的Vs可重复读取？

我目前正在尝试理解MySQL中的事务隔离,阅读本书High Performance MySQL, 2nd Edition.

以下是他们对这两个事务隔离级别的解释.

READ COMMITTED

The default isolation level for most database systems
(but not MySQL!) is READ COMMITTED . It satisfies the simple
definition of isolation used earlier: a transaction will see only
those changes made by transactions that were already committed when it
began, and its changes won’t be visible to others until it has
committed. This level still allows what’s known as a nonrepeatable
read. This means you can run the same statement twice and see
different data.

REPEATABLE READ

REPEATABLE READ solves the problems that READ
UNCOMMITTED allows. It guarantees that any rows a transaction reads
will “look the same” in subsequent reads within the same transaction,
but in theory it still allows another tricky problem: phantom reads.
Simply put, a phantom read can happen when you select some range of
rows, another transaction inserts a new row into the range, and then
you select the same range again; you will then see the new “phantom”
row. InnoDB and Falcon solve the phantom read problem with
multiversion concur- rency control, which we explain later in this
chapter. REPEATABLE READ is MySQL’s default transaction isolation
level. The InnoDB and Falcon storage engines respect this setting,
which you’ll learn how to change in Chapter 6. Some other storage
engines do too, but the choice is up to the engine.

问题：

1-在READ COMMITTED中,如果此隔离级别意味着事务只能看到其他事务所提交的更改,那么在同一事务期间如果运行相同的语句,您会看到不同的结果？
这是否意味着以下？

    START TRANSACTION;
                SELECT balance FROM checking WHERE customer_id = 10233276;
                UPDATE checking SET balance = balance - 200.00 WHERE customer_id = 10233276;
            # >>> NEXT I MUST SEE THE NEW BALANCE, OR I AM WRONG ? 
            SELECT balance FROM checking WHERE customer_id = 10233276;
COMMIT;

2-在REPEATABLE READ中,如果这个隔离级别允许幻读,那么为什么transactin读取的任何行将在后续读取中“看起来相同”？ phanton读取不反驳此级别的保证吗？

解决方法:

http://ronaldbradford.com/blog/understanding-mysql-innodb-transaction-isolation-2009-09-24/

可重复读

第1场会议：

    MariaDB [test]> DROP TABLE IF EXISTS transaction_test;
    Query OK, 0 rows affected (0.22 sec)

    MariaDB [test]> CREATE TABLE transaction_test(
        ->   id   INT UNSIGNED NOT NULL AUTO_INCREMENT,
        ->   val  VARCHAR(20) NOT NULL,
        ->   created TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
        -> PRIMARY KEY(id)
        -> ) ENGINE=InnoDB DEFAULT CHARSET latin1;
    Query OK, 0 rows affected (0.29 sec)

    MariaDB [test]>
    MariaDB [test]> INSERT INTO transaction_test(val) VALUES ('a'),('b'),('c');
    Query OK, 3 rows affected (0.08 sec)
    Records: 3  Duplicates: 0  Warnings: 0

    MariaDB [test]> START TRANSACTION;
    Query OK, 0 rows affected (0.00 sec)
MariaDB [test]> SELECT * FROM transaction_test;
+----+-----+---------------------+
| id | val | created             |
+----+-----+---------------------+
|  1 | a   | 2016-04-01 10:09:33 |
|  2 | b   | 2016-04-01 10:09:33 |
|  3 | c   | 2016-04-01 10:09:33 |
+----+-----+---------------------+
3 rows in set (0.00 sec)

MariaDB [test]> select sleep(50);

然后user2运行下一个代码：

 MariaDB [test]> START TRANSACTION;
Query OK, 0 rows affected (0.00 sec)

MariaDB [test]> INSERT INTO transaction_test(val) VALUES ('x'),('y'),('z');

commit;

那么用户1

MariaDB [test]> SELECT * FROM transaction_test;
+----+-----+---------------------+
| id | val | created             |
+----+-----+---------------------+
|  1 | a   | 2016-04-01 10:09:33 |
|  2 | b   | 2016-04-01 10:09:33 |
|  3 | c   | 2016-04-01 10:09:33 |
+----+-----+---------------------+
3 rows in set (0.00 sec)

MariaDB [test]>

读取已提交

USER1

SET SESSION tx_isolation='READ-COMMITTED';
TRUNCATE TABLE transaction_test;
INSERT INTO transaction_test(val) VALUES ('a'),('b'),('c');
MariaDB [test]> START TRANSACTION;
Query OK, 0 rows affected (0.00 sec)

MariaDB [test]> select sleep(60);

然后user2运行下一个代码：

MariaDB [test]> START TRANSACTION;
Query OK, 0 rows affected (0.00 sec)

MariaDB [test]> INSERT INTO transaction_test(val) VALUES ('x'),('y'),('zwfwfw');

Query OK, 3 rows affected (0.00 sec)
Records: 3  Duplicates: 0  Warnings: 0

MariaDB [test]> commit;

然后user1完成查询：

MariaDB [test]> SELECT * FROM transaction_test;
+----+--------+---------------------+
| id | val    | created             |
+----+--------+---------------------+
|  1 | a      | 2016-04-01 10:28:08 |
|  2 | b      | 2016-04-01 10:28:08 |
|  3 | c      | 2016-04-01 10:28:08 |
|  4 | x      | 2016-04-01 10:29:00 |
|  5 | 
y      | 2016-04-01 10:29:00 |
|  6 | zwfwfw | 2016-04-01 10:29:00 |
+----+--------+---------------------+
6 rows in set (0.00 sec)

标签：mysql,transactions,isolation-level
来源： https://codeday.me/bug/20190611/1220101.html