Database roles are conceptually completely separate from operating system users. In practice it might be convenient to maintain a correspondence, but this is not required. Database roles are global across a database cluster installation (and not per individual database). To create a role use the CREATE ROLE SQL command:
CREATE ROLE name;
name follows the rules for SQL identifiers: either unadorned without special characters, or double-quoted. (In practice, you will usually want to add additional options, such as LOGIN, to the command. More details appear below.) To remove an existing role, use the analogous DROP ROLE command:
DROP ROLE name;
createuser name dropuser name
To determine the set of existing roles, examine the pg_roles system catalog, for example
SELECT rolname FROM pg_roles;
The psql program's \du meta-command is also useful for listing the existing roles.
In order to bootstrap the database system, a freshly initialized system always contains one predefined role. This role is always a "superuser", and by default (unless altered when running initdb) it will have the same name as the operating system user that initialized the database cluster. Customarily, this role will be named postgres. In order to create more roles you first have to connect as this initial role.
Every connection to the database server is made using the name of some particular role, and this role determines the initial access privileges for commands issued in that connection. The role name to use for a particular database connection is indicated by the client that is initiating the connection request in an application-specific fashion. For example, the psql program uses the -U command line option to indicate the role to connect as. Many applications assume the name of the current operating system user by default (including createuser and psql). Therefore it is often convenient to maintain a naming correspondence between roles and operating system users.
The set of database roles a given client connection can connect as is determined by the client authentication setup, as explained in 19장. (Thus, a client is not limited to connect as the role matching its operating system user, just as a person's login name need not match her real name.) Since the role identity determines the set of privileges available to a connected client, it is important to carefully configure privileges when setting up a multiuser environment.