Question: How do you get the cluster ID in PostgreSQL?

Answer

PostgreSQL does not have a built-in concept of a 'cluster ID' in the same way that some other distributed databases might. In PostgreSQL, a 'cluster' typically refers to an instance of the PostgreSQL server process and its associated databases and configurations, but this cluster is not assigned a unique identifier like a UUID by default.

However, if you need to uniquely identify a PostgreSQL cluster, especially in environments where multiple clusters exist or in distributed settings, you would typically need to implement a custom solution. This could involve creating a configuration parameter or a dedicated table that stores a unique identifier for each cluster.

Here’s a simple example on how you can manually set and retrieve a cluster ID using a configuration parameter:

1. Setting a Cluster ID: You can use the ALTER SYSTEM command to add a custom configuration parameter. For instance, you could set a cluster ID as follows:

ALTER SYSTEM SET cluster.id TO '123e4567-e89b-12d3-a456-426614174000';

After running this command, make sure to reload the configuration:

SELECT pg_reload_conf();

2. Retrieving the Cluster ID: You can retrieve the cluster ID by querying the current settings:

SHOW cluster.id;

This will return the value you’ve set for cluster.id.

Remember, since PostgreSQL does not natively support cluster IDs, this approach is purely a custom implementation and should be tailored to fit your specific application requirements.