CREATE DATABASE actually works by copying an existing
   database.  By default, it copies the standard system database named
   template1. Thus that
   database is the “template” from which new databases are
   made.  If you add objects to template1, these objects
   will be copied into subsequently created user databases.  This
   behavior allows site-local modifications to the standard set of
   objects in databases.  For example, if you install the procedural
   language PL/Perl in template1, it will
   automatically be available in user databases without any extra
   action being taken when those databases are created.
  
   However, CREATE DATABASE does not copy database-level
   GRANT permissions attached to the source database.
   The new database has default database-level permissions.
  
   There is a second standard system database named
   template0. This
   database contains the same data as the initial contents of
   template1, that is, only the standard objects
   predefined by your version of
   PostgreSQL.  template0
   should never be changed after the database cluster has been
   initialized.  By instructing
   CREATE DATABASE to copy template0 instead
   of template1, you can create a “pristine” user
   database (one where no user-defined objects exist and where the system
   objects have not been altered) that contains none of the site-local additions in
   template1.  This is particularly handy when restoring a
   pg_dump dump: the dump script should be restored in a
   pristine database to ensure that one recreates the correct contents
   of the dumped database, without conflicting with objects that
   might have been added to template1 later on.
  
   Another common reason for copying template0 instead
   of template1 is that new encoding and locale settings
   can be specified when copying template0, whereas a copy
   of template1 must use the same settings it does.
   This is because template1 might contain encoding-specific
   or locale-specific data, while template0 is known not to.
  
   To create a database by copying template0, use:
CREATE DATABASE dbname TEMPLATE template0;
from the SQL environment, or:
createdb -T template0 dbname
from the shell.
   It is possible to create additional template databases, and indeed
   one can copy any database in a cluster by specifying its name
   as the template for CREATE DATABASE.  It is important to
   understand, however, that this is not (yet) intended as
   a general-purpose “COPY DATABASE” facility.
   The principal limitation is that no other sessions can be connected to
   the source database while it is being copied.  CREATE
   DATABASE will fail if any other connection exists when it starts;
   during the copy operation, new connections to the source database
   are prevented.
  
   Two useful flags exist in pg_database for each
   database: the columns datistemplate and
   datallowconn.  datistemplate
   can be set to indicate that a database is intended as a template for
   CREATE DATABASE.  If this flag is set, the database can be
   cloned by any user with CREATEDB privileges; if it is not set,
   only superusers and the owner of the database can clone it.
   If datallowconn is false, then no new connections
   to that database will be allowed (but existing sessions are not terminated
   simply by setting the flag false).  The template0
   database is normally marked datallowconn = false to prevent its modification.
   Both template0 and template1
   should always be marked with datistemplate = true.
  
    template1 and template0 do not have any special
    status beyond the fact that the name template1 is the default
    source database name for CREATE DATABASE.
    For example, one could drop template1 and recreate it from
    template0 without any ill effects.  This course of action
    might be advisable if one has carelessly added a bunch of junk in
    template1. (To delete template1,
    it must have pg_database.datistemplate = false.)
   
    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.