Guide to Using pg_prewarm

What is pg_prewarm?

pg_prewarm is a PostgreSQL module that allows you to load relation data into the PostgreSQL shared buffer cache or the operating system’s buffer cache before it is needed. This can be particularly useful after a database restart, as it helps to avoid the performance penalties typically associated with “cold” caches, where the data must be read from disk due to not being present in memory.

Use Cases for pg_prewarm

  • Improving Startup Performance: After a server restart, you can use pg_prewarm to quickly warm up the cache with the most frequently accessed data.
  • Preventing Latency Spikes: In scenarios where predictable performance is crucial, pg_prewarm can be used to ensure that data is in memory before high-load periods.
  • Benchmarking: When you want to measure the performance of queries without the overhead of disk I/O, pg_prewarm can be used to ensure that data is already in memory.

How to Use pg_prewarm

Before using pg_prewarm, ensure that the extension is installed and available in your PostgreSQL database. You can do this by running:


Here’s the basic syntax for the pg_prewarm command:

SELECT pg_prewarm('<relation>', mode := 'prefetch', fork := 'main');

In this command, <relation> is the name or OID of the table or index you want to prewarm, mode is the prewarm method, and fork specifies which fork of the relation to prewarm.

Examples of pg_prewarm in Action

Example 1: Prewarming a Single Table

To prewarm the entire ’employees’ table:

SELECT pg_prewarm('employees');

This command will load the ’employees’ table into the shared buffer cache.

Example 2: Prewarming Specific Forks

There are different forks like ‘main’, ‘fsm’, and ‘vm’ that you can prewarm. To prewarm the visibility map (vm) fork of the ’employees’ table:

SELECT pg_prewarm('employees', fork := 'vm');

Example 3: Prewarming Using Different Modes

pg_prewarm offers different modes like ‘prefetch’, ‘read’, and ‘buffer’. To prewarm the ’employees’ table using the ‘buffer’ mode (which loads blocks into the shared buffer):

SELECT pg_prewarm('employees', mode := 'buffer');

Common Mistakes and Issues

  • Extension Not Installed: The most common mistake is attempting to use pg_prewarm without installing the extension. Ensure it’s installed with the CREATE EXTENSION command.
  • Insufficient Shared Buffers: If your shared_buffers setting is too low, pg_prewarm may not be able to load all the desired data into the cache.
  • Incorrect Relation Names: Ensure that the relation names provided to pg_prewarm are correct. Typos or incorrect schema qualifiers can lead to errors.
  • Resource Consumption: Be cautious with pg_prewarm usage on large tables, as it can consume significant I/O and memory resources.

Troubleshooting and Errors

  • Permission Denied: The user executing pg_prewarm must have the necessary permissions to access the target relation.
  • Out of Memory: If the system runs out of memory while prewarming, consider prewarming in smaller chunks or increasing the system’s memory.
  • Cache Eviction: If other processes are actively using the database, they may evict prewarmed data from the cache. Schedule pg_prewarm during low-usage periods when possible.

By incorporating pg_prewarm into your database management practices, you can enhance the performance of your PostgreSQL server by ensuring that critical data is loaded into memory, ready for quick access. However, it’s important to use this tool judiciously to balance the benefits of a warm cache with the system’s overall resource availability.

Leave a Comment