Introduction to Mysql transaction isolation level content (read commit)

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Mysql transaction isolation level read commit

View mysql transaction isolation level

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

You can see that the current transaction isolation level is READ-COMMITTED read commit

Let’s look at the transaction isolation details under the current isolation level and open two query terminals A and B.

There is a order table below, the initial data is as follows

mysql> select * from `order`;
+----+--------+
| id | number |
+----+--------+
| 13 |      1 |
+----+--------+
1 row in set (0.00 sec)

The first step is to open the transaction in both A and B

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

The second step Query the number value in both terminals

• A

 mysql> select * from `order`;
+----+--------+
| id | number |
+----+--------+
| 13 |      1 |
+----+--------+
1 row in set (0.00 sec)

• B

 mysql> select * from `order`;
+----+--------+
| id | number |
+----+--------+
| 13 |      1 |
+----+--------+
1 row in set (0.00 sec)

The third step changes the number in B to 2, but does not commit the transaction

mysql> update `order` set number=2;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

The fourth step queries the value in A

mysql> select * from `order`;
+----+--------+
| id | number |
+----+--------+
| 13 |      1 |
+----+--------+
1 row in set (0.00 sec)

It is found that the value in A has not been modified.

The fifth step is to submit transaction B and query the value in A again

• B

mysql> commit;
Query OK, 0 rows affected (0.01 sec)

• A

mysql> select * from `order`;
+----+--------+
| id | number |
+----+--------+
| 13 |      2 |
+----+--------+
1 row in set (0.00 sec)

It is found that the value in A has changed

The sixth step is to submit the transaction in A and query the values ​​of A and B again.

• A

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

mysql> select * from `order`;
+----+--------+
| id | number |
+----+--------+
| 13 |      2 |
+----+--------+
1 row in set (0.00 sec)

• B

mysql> select * from `order`;
+----+--------+
| id | number |
+----+--------+
| 13 |      2 |
+----+--------+
1 row in set (0.00 sec)

Found A and B The values ​​have been changed to 2.

The following is a simple diagram

We can see that the transaction isolation level is Read Committed In the case of B, after the transaction in B is submitted, the result of B transaction submission can be read even if A has not submitted. This solves the problem of dirty reading.

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