Every database cluster contains a set of database users. Those users are separate from the users managed by the operating system on which the server runs. Users own database objects (for example, tables) and can assign privileges on those objects to other users to control who has access to which object.
This chapter describes how to create and manage users and introduces the privilege system. More information about the various types of database objects and the effects of privileges can be found in Chapter 5.
Database users are conceptually completely separate from operating system users. In practice it might be convenient to maintain a correspondence, but this is not required. Database user names are global across a database cluster installation (and not per individual database). To create a user use the CREATE USER SQL command:
CREATE USER name;
name follows the rules for SQL identifiers: either unadorned without special characters, or double-quoted. To remove an existing user, use the analogous DROP USER command:
DROP USER name;
createuser name dropuser name
To determine the set of existing users, examine the pg_user system catalog, for example
SELECT usename FROM pg_user;
The psql program's \du meta-command is also useful for listing the existing users.
In order to bootstrap the database system, a freshly initialized system always contains one predefined user. This user will have the fixed ID 1, 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 user will be named postgres. In order to create more users you first have to connect as this initial user.
Exactly one user identity is active for a connection to the database server. The user 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 user to connect as. Many applications assume the name of the current operating system user by default (including createuser and psql). Therefore it is convenient to maintain a naming correspondence between the two user sets.
The set of database users a given client connection may connect as is determined by the client authentication setup, as explained in Chapter 19. (Thus, a client is not necessarily limited to connect as the user with the same name as its operating system user, just as a person's login name need not match her real name.) Since the user identity determines the set of privileges available to a connected client, it is important to carefully configure this when setting up a multiuser environment.