I. Introduction

In PostgreSQL, there are two types of tables that can be used to store data: unlogged tables and logged tables. Each type of table has its own characteristics and use cases, depending on the requirements of the application. In this article, we will explore the differences between unlogged tables and logged tables in PostgreSQL and discuss when to use each type of table for optimal performance.

II. Unlogged Tables

1. Definition

Unlogged tables in PostgreSQL are tables that do not write any data to the write-ahead log (WAL). This means that changes made to unlogged tables are not replicated to the standby servers and are not recoverable in case of a crash or failure.

2. Characteristics

  • Changes made to unlogged tables are not WAL-logged, which can improve performance for write-heavy workloads.
  • Unlogged tables are not crash-safe, as changes made to them are not replicated to the standby servers.
  • Unlogged tables are not suitable for storing critical or important data, as they are not recoverable in case of a crash or failure.

3. Use cases in real world

Unlogged tables are suitable for storing temporary or transient data that does not need to be replicated or recovered. They are commonly used for storing session data, temporary data, or any data that can be easily recreated or regenerated in case of a failure.

Caching data is a common use case for unlogged tables. For example, you might use an unlogged table to store the results of a computationally expensive query that can be easily recalculated if the data is lost. By using an unlogged table for caching, you can improve performance by avoiding the overhead of WAL logging.

III. Logged Tables

1. Definition

Logged tables in PostgreSQL are tables that write all changes to the write-ahead log (WAL). This means that changes made to logged tables are replicated to the standby servers and are recoverable in case of a crash or failure.

2. Characteristics

  • Changes made to logged tables are WAL-logged, which ensures data consistency and durability.
  • Logged tables are crash-safe, as changes made to them are replicated to the standby servers and can be recovered in case of a crash or failure.
  • Logged tables are suitable for storing critical or important data that needs to be replicated and recovered in case of a failure.

3. Use cases in real world

Logged tables are suitable for storing critical or important data that needs to be replicated and recovered in case of a failure. They are commonly used for storing transactional data, audit logs, or any data that needs to be preserved and recovered in case of a crash or failure.

IV. Comparison

1. Performance

  • Unlogged tables can offer better performance for write-heavy workloads, as changes are not WAL-logged.
  • Logged tables provide data consistency and durability, but may have slightly lower performance due to WAL logging.

2. Data Durability

  • Unlogged tables are not crash-safe and do not provide data durability, as changes are not replicated to the standby servers.
  • Logged tables are crash-safe and provide data durability, as changes are replicated to the standby servers and can be recovered in case of a crash or failure.

IV. Conclusion

In PostgreSQL, unlogged tables and logged tables serve different purposes and have different characteristics. Unlogged tables are suitable for storing temporary or transient data that does not need to be replicated or recovered, while logged tables are suitable for storing critical or important data that needs to be replicated and recovered in case of a failure. By understanding the differences between unlogged tables and logged tables, you can choose the appropriate table type for your application to achieve optimal performance and data durability.