Is the "postgres" database always available in PostgreSQL?

Question

I'm writing a script that needs to get all the databases in a postgres cluster, and so it needs to connect to a database (it doesn't matter which) to do this.

Is it okay to assume that the "postgres" database will always be available to connect to, or if not, is there a programatic way to determine an arbitrary database in the cluster that I can connect to?

Answer 1

PostgreSQL has three "system" databases: postgres, template0, and template1. I'm pretty sure all of them can be dropped. (You might want to test this in a virtual machine.) I'm certain that postgres and template1 can be dropped by a privileged user. By default, template0 doesn't accept connections.

Having said that, the main purpose of "postgres" is to give you a database to connect to.

The postgres database is also created when a database cluster is initialized. This database is meant as a default database for users and applications to connect to. It is simply a copy of template1 and can be dropped and recreated if necessary.

So it's a reasonable assumption, but not a bullet-proof assumption. I can imagine some paranoid (careful) shops might drop "postgres" and use a different database as their internal default in case their server were compromised. That's kind of like using a non-default port for SSH.