MySQL MVCC principle analysis and application practice: improving database transaction processing efficiency

MySQL MVCC principle analysis and application practice: improving database transaction processing efficiency

1. MVCC principle analysis

MVCC (Multi-Version Concurrency Control) It is a mechanism to achieve concurrency control in MySQL. It achieves the isolation of concurrent transactions by recording historical versions of rows, avoiding lock contention and blocking. The implementation of MVCC mainly relies on version chain and read view.

1. Version Chain

Whenever a transaction modifies the database, MySQL will create a new version for each row of data. These versions are linked together to form a version chain. At the beginning of a transaction, MySQL will create a "read view" for the transaction. The read view will record the starting point of the version chain when the transaction is started.

2. Read view

Read view is the key to transaction isolation level. It defines which data versions the transaction can see. The read view records the starting point of the version chain when the transaction is started, and will change as the data is modified during the transaction execution. Read views ensure that a transaction can only see the version of the data that was committed before it was started.

When a transaction wants to read data, it will select the appropriate data version from the version chain based on its own read view. If the version was created by a transaction that has not yet committed, MySQL determines whether the transaction can read the data in that version based on the transaction's commit status.

2. Application Practice

In actual development, understanding and using MVCC can effectively improve the transaction processing efficiency of the database. The following takes a simple application scenario as an example to introduce how to use MVCC.

Suppose there is a user table (user), which contains three fields: id, name and age. We want to get the user records whose age is greater than 20 in this table.

1. Create a test table

First, we need to create a test table and insert some test data.

CREATE TABLE user (
    id INT PRIMARY KEY,
    name VARCHAR(50),
    age INT
);

INSERT INTO user (id, name, age) VALUES (1, 'Alice', 18);
INSERT INTO user (id, name, age) VALUES (2, 'Bob', 25);
INSERT INTO user (id, name, age) VALUES (3, 'Charlie', 30);
INSERT INTO user (id, name, age) VALUES (4, 'David', 22);
INSERT INTO user (id, name, age) VALUES (5, 'Eve', 28);

2. Use MVCC to query data

Next, we use MVCC to query user records that meet the conditions.

START TRANSACTION; -- 开启事务

-- 设置事务的隔离级别为可重复读
SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;

-- 创建读视图
SELECT * FROM user WHERE age > 20;

The above SQL statement will return user records with age greater than 20, but only in the data version that existed when the current transaction was started. If other transactions modify the records of the data table during transaction execution, these modifications are not visible to the current transaction.

3. Modify data and submit transaction

At the same time as the above query operation, we can modify the records of the data table in another transaction.

START TRANSACTION; -- 开启事务

UPDATE user SET age = 21 WHERE id = 1;

COMMIT; -- 提交事务

After the data modification transaction is submitted, if you execute the above query operation again, you will get the updated results.

Through the above practical application examples, we can see the advantages of MVCC. Using MVCC can avoid locking operations on data rows and reduce the impact on concurrent transactions, thus improving the transaction processing efficiency of the database.

3. Summary

MVCC is a mechanism for MySQL to implement concurrency control. Through version chains and read views, MVCC achieves transaction isolation and avoids lock contention and blocking. In actual development, reasonable application of MVCC can improve database transaction processing efficiency. Therefore, it is very important for MySQL developers to have an in-depth understanding and proficiency in using MVCC.

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