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 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 "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 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
.
Note:
template1
andtemplate0
do not have any special status beyond the fact that the nametemplate1
is the default source database name forCREATE DATABASE
. For example, one could droptemplate1
and recreate it fromtemplate0
without any ill effects. This course of action might be advisable if one has carelessly added a bunch of junk intemplate1
. (To deletetemplate1
, it must havepg_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 oftemplate1
and can be dropped and recreated if necessary.
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