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Chapter 6. Managing Databases

A database is a named collection of SQL objects ("database objects"). Generally, every database object (tables, functions, etc.) belongs to one and only one database. (But there are a few system catalogs, for example pg_database, that belong to a whole installation and are accessible from each database within the installation.) An application that connects to the database server specifies in its connection request the name of the database it wants to connect to. It is not possible to access more than one database per connection. (But an application is not restricted in the number of connections it opens to the same or other databases.)

Note: SQL calls databases "catalogs", but there is no difference in practice.

In order to create or drop databases, the PostgreSQL postmaster must be up and running (see Section 3.3).

6.1. Creating a Database

Databases are created with the query language command CREATE DATABASE:

CREATE DATABASE name

where name follows the usual rules for SQL identifiers. The current user automatically becomes the owner of the new database. It is the privilege of the owner of a database to remove it later on (which also removes all the objects in it, even if they have a different owner).

The creation of databases is a restricted operation. See Section 7.1.1 for how to grant permission.

Bootstrapping: Since you need to be connected to the database server in order to execute the CREATE DATABASE command, the question remains how the first database at any given site can be created. The first database is always created by the initdb command when the data storage area is initialized. (See Section 3.2.) By convention this database is called template1. So to create the first "real" database you can connect to template1.

The name "template1" is no accident: When a new database is created, the template database is essentially cloned. This means that any changes you make in template1 are propagated to all subsequently created databases. This implies that you should not use the template database for real work, but when used judiciously this feature can be convenient. More details appear below.

As an extra convenience, there is also a program that you can execute from the shell to create new databases, createdb.

createdb dbname

createdb does no magic. It connects to the template1 database and issues the CREATE DATABASE command, exactly as described above. It uses the psql program internally. The reference page on createdb contains the invocation details. Note that createdb without any arguments will create a database with the current user name, which may or may not be what you want.

6.1.1. 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 plpgsql 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.

It is possible to create additional template databases, and indeed one might copy any database in an installation 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. In particular, it is essential that the source database be idle (no data-altering transactions in progress) for the duration of the copying operation. CREATE DATABASE will check that no backend processes (other than itself) are connected to the source database at the start of the operation, but this does not guarantee that changes cannot be made while the copy proceeds, which would result in an inconsistent copied database. Therefore, we recommend that databases used as templates be treated as read-only.

Two useful flags exist in pg_database for each database: 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.

After preparing a template database, or making any changes to one, it is a good idea to perform VACUUM FREEZE or VACUUM FULL FREEZE in that database. If this is done when there are no other open transactions in the same database, then it is guaranteed that all tuples in the database are "frozen" and will not be subject to transaction ID wraparound problems. This is particularly important for a database that will have datallowconn set to false, since it will be impossible to do routine maintenance VACUUMs on such a database. See Section 8.2.3 for more information.

Note: 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 and the default database-to-connect-to for various scripts such as createdb. 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.

6.1.2. Alternative Locations

It is possible to create a database in a location other than the default location for the installation. Remember that all database access occurs through the database server, so any location specified must be accessible by the server.

Alternative database locations are referenced by an environment variable which gives the absolute path to the intended storage location. This environment variable must be present in the server's environment, so it must have been defined before the server was started. (Thus, the set of available alternative locations is under the site administrator's control; ordinary users can't change it.) Any valid environment variable name may be used to reference an alternative location, although using variable names with a prefix of PGDATA is recommended to avoid confusion and conflict with other variables.

To create the variable in the environment of the server process you must first shut down the server, define the variable, initialize the data area, and finally restart the server. (See Section 3.6 and Section 3.3.) To set an environment variable, type

PGDATA2=/home/postgres/data
export PGDATA2
in Bourne shells, or
setenv PGDATA2 /home/postgres/data
in csh or tcsh. You have to make sure that this environment variable is always defined in the server environment, otherwise you won't be able to access that database. Therefore you probably want to set it in some sort of shell start-up file or server start-up script.

To create a data storage area in PGDATA2, ensure that the containing directory (here, /home/postgres) already exists and is writable by the user account that runs the server (see Section 3.1). Then from the command line, type

initlocation PGDATA2
Then you can restart the server.

To create a database within the new location, use the command

CREATE DATABASE name WITH LOCATION = 'location'

where location is the environment variable you used, PGDATA2 in this example. The createdb command has the option -D for this purpose.

Databases created in alternative locations can be accessed and dropped like any other database.

Note: It can also be possible to specify absolute paths directly to the CREATE DATABASE command without defining environment variables. This is disallowed by default because it is a security risk. To allow it, you must compile PostgreSQL with the C preprocessor macro ALLOW_ABSOLUTE_DBPATHS defined. One way to do this is to run the compilation step like this:

gmake CPPFLAGS=-DALLOW_ABSOLUTE_DBPATHS all
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