22.3. Template Databases
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.
There is a second standard system database named
database contains the same data as the initial contents of
template1, that is, only the standard objects
predefined by your version of
should never be changed after the database cluster has been
initialized. By instructing
CREATE DATABASE to copy
template1, you can create a “virgin” user
database 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
virgin 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
template1 is that new encoding and locale settings
can be specified when copying
template0, whereas a copy
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
from the SQL environment, or:
createdb -T template0
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.
DATABASE will fail if any other connection exists when it starts;
during the copy operation, new connections to the source database
Two useful flags exist in
pg_database for each
database: the columns
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.
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
database is normally marked
datallowconn = false to prevent its modification.
should always be marked with
datistemplate = true.
template0 do not have any special
status beyond the fact that the name
template1 is the default
source database name for
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
it must have
pg_database.datistemplate = false.)
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.