I. Introduction

MVCC (Multi-Version Concurrency Control) is a technique used in databases like MySQL and PostgreSQL to manage concurrent access to the database. MVCC allows multiple transactions to read and write data simultaneously without blocking each other, providing a high level of concurrency and performance.

In this article, we will explore how MVCC works in databases like MySQL and PostgreSQL and its benefits for developers and users.

II. How MVCC Works in MySQL and PostgreSQL

MVCC works by creating multiple versions of a row in the database to represent different states of the data at different points in time. When a transaction updates a row, the database creates a new version of the row with the updated data, while keeping the old version intact. This allows other transactions to continue reading the old version of the row without being affected by the update.

When a transaction commits or rolls back, the database marks the old version of the row as obsolete and removes it from the database. This process is known as garbage collection and helps the database maintain a clean and efficient database.

MVCC uses a combination of read and write locks to ensure that transactions can read and write data concurrently without interfering with each other. Read locks allow transactions to read data without blocking other transactions, while write locks prevent multiple transactions from updating the same row simultaneously.

III. Differences in storing MVCC data in MySQL and PostgreSQL

While both MySQL and PostgreSQL use MVCC to manage concurrent access to the database, there are some differences in how they store MVCC data.

In MySQL, MVCC data is stored in the undo log, which is a separate storage area that contains the old versions of rows. When a transaction updates a row, MySQL creates a new version of the row in the undo log and updates the row in the main table. This allows other transactions to read the old version of the row from the undo log.

In PostgreSQL, MVCC data is stored in the heap and the visibility map. The heap contains the current version of the row, while the visibility map contains information about which rows are visible to which transactions. When a transaction updates a row, PostgreSQL creates a new version of the row in the heap and updates the visibility map to reflect the changes. This allows other transactions to read the old version of the row until the transaction commits or rolls back.

IV. Benefits of these approaches in MySQL and PostgreSQL

Both MySQL and PostgreSQL use MVCC to manage concurrent access to the database, but they store MVCC data differently. Each approach has its benefits and trade-offs.

In MySQL, storing MVCC data in the undo log allows for faster rollback of transactions and better performance for write-heavy workloads. However, storing MVCC data in a separate storage area can lead to increased storage overhead and slower read performance.

In PostgreSQL, storing MVCC data in the heap and visibility map allows for better read performance and lower storage overhead. However, updating the visibility map can introduce additional overhead for write-heavy workloads.

V. Pros and Cons of MVCC in PostgreSQL compared to MySQL

While both MySQL and PostgreSQL use MVCC to manage concurrent access to the database, there are some pros and cons to consider when choosing between the two databases.

1. PostgreSQL

Pros of MVCC in PostgreSQL:

  • Better read performance: Storing MVCC data in the heap and visibility map allows for faster read performance in PostgreSQL.
  • Lower storage overhead: Storing MVCC data in the heap and visibility map can reduce storage overhead in PostgreSQL.

Cons of MVCC in PostgreSQL:

  • Higher write overhead: Updating the visibility map can introduce additional overhead for write-heavy workloads in PostgreSQL.

2. MySQL

Pros of MVCC in MySQL:

  • Faster rollback of transactions: Storing MVCC data in the undo log allows for faster rollback of transactions in MySQL.

Cons of MVCC in MySQL:

  • Increased storage overhead: Storing MVCC data in a separate storage area can lead to increased storage overhead in MySQL.

VI. Which and when to use MySQL or PostgreSQL

When choosing between MySQL and PostgreSQL for your database needs, consider the pros and cons of each approach to storing MVCC data.

Use MySQL if:

  • You have write-heavy workloads that require fast rollback of transactions.
  • You are willing to trade read performance for better write performance.

Use PostgreSQL if:

  • You have read-heavy workloads that require fast read performance.
  • You are willing to trade write performance for better read performance and lower storage overhead.

VI. Conclusion

MVCC is a powerful technique used in databases like MySQL and PostgreSQL to manage concurrent access to the database. By creating multiple versions of rows and using read and write locks, databases provide a high level of concurrency and performance for developers and users. While both MySQL and PostgreSQL use MVCC to manage concurrent access, they store MVCC data differently, each with its benefits and trade-offs. Developers and users should consider the pros and cons of each approach when choosing between MySQL and PostgreSQL for their database needs.