21.3. 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
template0. This database contains the same data as the initial contents of
template1, that is, only the standard objects predefined by your version of Postgres Pro.
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
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.
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 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
template1 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
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.)
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.