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/pgSQL in template1
, it will
automatically be available in user databases without any extra
action being taken when those databases are made.
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 initdb
. By instructing
CREATE DATABASE
to copy template0
instead
of 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 any conflicts with additions that
may now be present in template1
.
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 may 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;
otherwise, new connections to the source database are locked out
until CREATE DATABASE
completes.
Two useful flags exist in pg_database
for each
database: the columns datistemplate
and
datallowconn
. datistemplate
may be set to indicate that a database is intended as a template for
CREATE DATABASE
. If this flag is set, the database may be
cloned by
any user with CREATEDB
privileges; if it is not set, only superusers
and the owner of the database may clone it.
If datallowconn
is false, then no new connections
to that database will be allowed (but existing sessions are not killed
simply by setting the flag false). The template0
database is normally marked datallowconn = false
to prevent modification of it.
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 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 may be dropped and recreated if required.