Create Database Uses Template1

Whenever you use the create database query, unless otherwise specified, it will create it by cloning template1 by default.

You can view, inspect, and even modify this database template by connecting to it.

\c template1

Every Postgres cluster starts with two templates.

select datname from pg_database where datistemplate = true;
  datname
-----------
 template1
 template0
(2 rows)

You cannot however connect to and modify template0. It is a fallback clone of template1 that you can utilize if you ever modify template1 and need to restore it.

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