In-depth interpretation of MySQL MVCC principles and best practices

In-depth interpretation of MySQL MVCC principles and best practices

1. Overview
MySQL is one of the most widely used relational database management systems, and it supports many Version concurrency control (Multi-Version Concurrency Control, MVCC) mechanism to handle concurrent access issues. This article will provide an in-depth explanation of the principles of MySQL MVCC and give some best practice examples.

2. MVCC principle

1. Version number
   MVCC is implemented by adding an additional version number to each data row. Each time a data row is modified, a new version number is generated for each modified version.
2. Transaction ID
   In MVCC, each transaction has a unique transaction ID (Transaction ID). There are many ways to generate and assign transaction IDs, such as based on timestamps or based on sequence number generators.
3. Version control of data rows
   Each data row will save the version number and expired version number when it was created. The creation version number indicates the transaction in which the data row of this version was created, and the expired version number indicates the transaction in which the data row of this version expired or was deleted.
4. Transaction read operation
   When a transaction reads a data row, it will make a judgment based on the transaction ID of the transaction itself and the version information of the data row. A data row is visible if its creation version number is earlier than the transaction's start ID and its expired version number is later than the transaction's start ID. On the contrary, if the creation version number of the data row is later than the start ID of the transaction, or the expired version number is earlier than the start ID of the transaction, the data row is invisible.
5. Transaction write operation
   In MVCC, each transaction's write operation on a data row actually creates a new version of the data row and updates the expired version number of the data row. . In this way, as long as the creation version number of the data row is earlier than the start ID of the transaction, and the expired version number is later than the start ID of the transaction, it is guaranteed that the modification of the data row by the transaction will not affect the read operations of other transactions.

3. MVCC best practices

1. Avoid long-term read transactions
   Long-term read transactions may cause the MVCC version chain to be too long, thus occupying a large amount of storage space. Minimize the existence of long read transactions and include read operations in a shorter transaction whenever possible.
2. Increase the innodb_undo_log_truncate parameter appropriately
   The innodb_undo_log_truncate parameter is used to control the recycling process of the MVCC version chain. If the version chain is too long, the recycling operation will be inefficient. You can increase the value of this parameter appropriately so that the recycling operation can be performed in a more timely manner.
3. Set the innodb_max_purge_lag parameter appropriately
   The innodb_max_purge_lag parameter is used to control the cleaning process of the MVCC version chain. When a large number of transactions are submitted, if the version chain cannot be cleaned up in time, a large amount of storage space will be occupied. Set the value of this parameter appropriately so that the cleanup process can keep up with the speed of transaction submission.

The following is a MySQL MVCC sample code:

-- 创建测试表
CREATE TABLE test (
    id INT PRIMARY KEY,
    value VARCHAR(50)
) ENGINE=InnoDB;

-- 开启事务 A
START TRANSACTION;

-- 向测试表中插入一条数据
INSERT INTO test (id, value) VALUES (1, 'Test');

-- 开启事务 B
START TRANSACTION;

-- 查询测试表
SELECT * FROM test;

-- 向测试表中插入一条数据
INSERT INTO test (id, value) VALUES (2, 'Test');

-- 提交事务 B
COMMIT;

-- 向测试表中插入一条数据
INSERT INTO test (id, value) VALUES (3, 'Test');

-- 提交事务 A
COMMIT;

-- 查询测试表
SELECT * FROM test;

Through the above sample code, we can observe the impact of read and write operations on data in different transactions. Data rows inserted by transaction A are not visible to transaction B before the start, and data rows inserted by transaction B are not visible to transaction A after the start.

Summary:
MySQL MVCC is a mechanism to implement concurrency control by adding a version number to each data row. Understanding its principles is very important to improve the concurrent access performance of the database. In actual applications, it is necessary to set relevant parameters according to the actual situation and follow some best practices to better utilize the MVCC mechanism to optimize database operations.

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