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Posted by: AJ Welch
Modifying user permissions within PostgreSQL can range from rather simple to extremely complex, depending on the permissive granularity that is actually required. In most cases, however, it is the powerful ALTER USER command that should be utilized to do everything from allowing users to login, create databases, manage roles, and even become a SUPERUSER account.
We’ll briefly explore the power of the ALTER USER command so you can easily perform a variety of permission assignments and removals as the need arises.
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Before we get into altering user permissions, we should establish a new user account (commonly referred to as a ROLE) to mess around with.
To begin, we’ll list all the existing users:
=# SELECT usename FROM pg_user;
usename
----------
postgres
(1 row)By default, postgres is typically the only user that exists, so we want to create a new user of librarian to control our library database. This can be accomplished using the CREATE USER command:
=# CREATE USER librarian;
CREATE ROLE
=# SELECT usename FROM pg_user;
usename
-----------
postgres
librarian
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It can often be useful to examine the existing permissions assigned to the users in the system. This can easily be accomplished with the \du command from the psql prompt:
=# \du
List of roles
Role name | Attributes | Member of
-----------+------------------------------------------------+-----------
librarian | | {}
postgres | Superuser, Create role, Create DB, Replication | {}We can clearly see that even though we’ve now added a new librarian user, we have to assign it some permissions.
Altering existing user permissions
Now that our librarian user exists, we can begin using ALTER USER to modify the permissions granted to librarian.
The basic format of ALTER USER includes the name of the user (or ROLE) followed by a series of options to inform PostgreSQL which permissive alterations to make:
=# ALTER USER role_specification WITH OPTION1 OPTION2 OPTION3;These options range from CREATEDB, CREATEROLE, CREATEUSER, and even SUPERUSER. Additionally, most options also have a negative counterpart, informing the system that you wish to deny the user that particular permission. These option names are the same as their assignment counterpart, but are prefixed with NO (e.g. NOCREATEDB, NOCREATEROLE, NOSUPERUSER).
Assigning SUPERUSER permission
Now that we understand the basics of creating users and using ALTER USER to modify permissions, we can quite simply use the SUPERUSER option to assign our librarian user SUPERUSER permission:
=# ALTER USER librarian WITH SUPERUSER;
ALTER ROLESure enough, if we display our permission list now, we’ll see librarian has the new SUPERUSER permission we want:
=# \du
List of roles
Role name | Attributes | Member of
-----------+------------------------------------------------+-----------
librarian | Superuser | {}
postgres | Superuser, Create role, Create DB, Replication | {}Revoking permissions
In the event that we make a mistake and assign a permission we later wish to revoke, simply issue the same ALTER USER command but add the NO prefix in front of the permissive options to be revoked.
For example, we can remove SUPERUSER from our librarian user like so:
=# ALTER USER librarian WITH NOSUPERUSER;
ALTER ROLE
=# \du
List of roles
Role name | Attributes | Member of
-----------+------------------------------------------------+-----------
librarian | | {}
postgres | Superuser, Create role, Create DB, Replication | {}