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 isREAD-COMMITTEDread commit

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

There is aordertable 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 thenumbervalue 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 thenumberin 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 isRead CommittedIn 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 ofdirty reading.

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