Once PostgreSQL is running, it awaits incoming database connections to serve; as soon as a connection comes in, PostgreSQL serves it by connecting the client to the right database. This means that in order to interact with the cluster, you need to connect to it. However, you don't connect to the whole cluster; rather, you ask PostgreSQL to interact with one of the databases the cluster is serving. Therefore, when you connect to the cluster, you need to connect to a specific database. This also means that the cluster must have at least one database from the very beginning of its life. That is the role of the so-called template databases, which, among other duties, serve as a common database to which you can connect on a freshly installed cluster.
When you initialize the cluster with the initdb command, PostgreSQL builds the filesystem layout of the PGDATA directory and builds two template databases, named template0 and template1. The aim of these databases...
