Detailed explanation of MySQL MVCC principle and its impact on database performance

MySQL is an open source relational database management system that is widely used in the development of Web applications. One of the important features is the MVCC (Multi-Version Concurrency Control, multi-version concurrency control) mechanism. This article will analyze the principle of MVCC in MySQL in detail and analyze its impact on database performance.

1. What is MVCC

MVCC is a concurrency control technology used to solve the problems caused by concurrent operations of multiple transactions reading and writing to the database at the same time. Traditional concurrency control technologies, such as lock mechanisms, can easily cause blocking and deadlock between transactions. MVCC achieves a high degree of concurrency by creating an independent data version for each transaction so that each transaction can see a logically consistent database snapshot.

2. Implementation principle of MVCC

1. Data version number

In MVCC, each data row has a hidden version number. Whenever a transaction performs an UPDATE or DELETE operation on a data row, a new version of the data row is created and the old version of the data is marked as expired.

2. Transaction read rules

In MVCC, for a transaction, it can only read the version of data that existed before it started. That is, a transaction can only see the version of the data row committed before it started.

3. Data cleaning

In order to avoid performance degradation caused by too many versions in the database, MySQL will regularly perform data version cleaning operations. This operation will delete the expired version to free up space.

3. The impact of MVCC on database performance

1. Read and write concurrency performance

Because MVCC can make transactions execute concurrently without blocking each other, Therefore, the read and write concurrency performance of the database is greatly improved. In the case of high concurrency, the traditional lock mechanism can easily lead to deadlock and blocking, while MVCC solves this problem through version isolation.

2. Storage space consumption

The MVCC mechanism needs to maintain multiple versions of information for each data row, so it will increase the storage space consumption of the database. Especially when there are a large number of long-running transactions in the database, the storage overhead of versions will be more obvious. Therefore, storage space consumption needs to be fully considered when designing the database.

3. Performance overhead of cleaning operations

MVCC needs to regularly perform data version cleaning operations. This operation will perform high-load scanning and deletion operations on the database, so it will bring certain performance overhead. Especially when there are a large number of versions in the database, the performance problem of the cleanup operation will be more prominent.

4. Precautions for using MVCC

1. Properly control the concurrency of transactions

Although MVCC can improve the concurrency performance of the database, it also needs to be properly controlled Control the concurrency of transactions. When concurrency is too high, it will lead to too many versions, thereby increasing the overhead of cleanup operations and the consumption of storage space.

2. Perform cleanup operations regularly

In order to avoid excessive consumption of database storage space, cleanup operations need to be performed regularly. The frequency of cleaning operations and the size of batch operations can be adjusted according to actual conditions.

3. Rationally design the database table structure

When designing the database table structure, you can consider moving frequently updated fields to separate tables to reduce the number of versions. This can effectively reduce storage space consumption and cleanup operation overhead.

5. Summary

MVCC is an important feature in MySQL, which achieves concurrency control by creating versions of data rows. It can improve the read and write concurrency performance of the database, but it will also cause storage space consumption and performance overhead of cleanup operations. When using MVCC, you need to properly control the concurrency of transactions, perform cleanup operations regularly, and properly design the database table structure. Only by fully understanding its principles and precautions can we better use MVCC to improve database performance.

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