In-depth analysis of MySQL MVCC principle and implementation

In-depth analysis of the principle and implementation of MySQL MVCC

MySQL is one of the most popular relational database management systems currently. It provides multiversion concurrency control (Multiversion Concurrency Control , MVCC) mechanism to support efficient concurrent processing. MVCC is a method of handling concurrent transactions in the database that can provide high concurrency and isolation.

This article will provide an in-depth analysis of the principles and implementation of MySQL MVCC, and illustrate it with code examples.

1. MVCC principle

MVCC is implemented based on the row-level locking mechanism of the database. Each transaction will generate a unique transaction ID when executed, called Transaction ID (TID for short).

In MVCC, multiple versions of each data row are stored. When a transaction modifies a data row, a new data row version will be generated, and this version will save the transaction ID, indicating that the version was generated by the transaction.

When reading data, each transaction can only see the data row versions produced by transactions that have been committed before its start time, and cannot see modifications that have not been committed by other transactions.

When deleting data, MySQL will generate a delete mark and does not actually delete the data rows. This is to ensure that read operations are not affected by ongoing delete operations.

2. MVCC Implementation

In MySQL, each data row will have three fields to save version information: create version number (Create Version), delete version number (Delete Version), before A version number (Previous Version).

The created version number is used to record the time when the transaction started, and the deleted version number is used to record the time when the transaction was submitted. The previous version number points to the data row of the previous version.

The following uses sample code to illustrate how MVCC is implemented.

-- 创建测试表
CREATE TABLE `student` (
  `id` INT NOT NULL AUTO_INCREMENT,
  `name` VARCHAR(50) NOT NULL,
  `age` INT NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB;

-- 插入测试数据
INSERT INTO student (name, age) VALUES ('Alice', 20), ('Bob', 21);

-- 打开事务1，插入一条数据
START TRANSACTION;
INSERT INTO student (name, age) VALUES ('Charlie', 22);

-- 打开事务2，读取数据，此时只能看到事务1之前的数据
START TRANSACTION;
SELECT * FROM student;
-- 输出：(1, 'Alice', 20), (2, 'Bob', 21)

-- 提交事务1，释放事务1的锁
COMMIT;

-- 在事务2中再次读取
SELECT * FROM student;
-- 输出：(1, 'Alice', 20), (2, 'Bob', 21), (3, 'Charlie', 22)

-- 关闭事务2
COMMIT;

-- 删除数据，实际上是生成一个删除标记
START TRANSACTION;
DELETE FROM student WHERE id = 2;

-- 打开事务3，读取数据，此时只能看到事务3之前的数据
START TRANSACTION;
SELECT * FROM student;
-- 输出：(1, 'Alice', 20), (3, 'Charlie', 22)

-- 提交事务3，数据行被删除
COMMIT;

-- 在事务4中再次读取
SELECT * FROM student;
-- 输出：(1, 'Alice', 20), (3, 'Charlie', 22)

In the above example, transaction 1 inserts a piece of data, and transaction 2 can only see the data before transaction 1 before transaction 1 commits. Transaction 3 deletes a data row and generates a deletion mark. Transaction 4 can only see the data before transaction 3 before transaction 3 commits.

Through the MVCC mechanism, different transactions can read and modify the database concurrently, improving the concurrency performance and isolation of the database.

3. Summary

MVCC is one of the key mechanisms for MySQL to achieve high concurrency and isolation. By recording version information of data rows, MySQL can provide isolated read and write operations between different transactions. At the same time, the implementation of MVCC also brings some additional overhead, such as storing additional version information and processing deletion operations.

Understanding the principles and implementation of MVCC can help developers make better use of MySQL's concurrency control mechanism and design high-performance database applications.

The above is the detailed content of In-depth analysis of MySQL MVCC principle and implementation. For more information, please follow other related articles on the PHP Chinese website!