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Note: The following description applies both to Postgres-XC and PostgreSQL if not described explicitly.
A database role can have a number of attributes that define its privileges and interact with the client authentication system.
Only roles that have the LOGIN attribute can be used as the initial role name for a database connection. A role with the LOGIN attribute can be considered the same as a "database user". To create a role with login privilege, use either:
CREATE ROLE name LOGIN; CREATE USER name;
(CREATE USER is equivalent to CREATE ROLE except that CREATE USER assumes LOGIN by default, while CREATE ROLE does not.)
A database superuser bypasses all permission checks, except the right to log in or the right to initiate replication. This is a dangerous privilege and should not be used carelessly; it is best to do most of your work as a role that is not a superuser. To create a new database superuser, use CREATE ROLE name SUPERUSER. You must do this as a role that is already a superuser. Creating a superuser will by default also grant permissions to initiate streaming replication. For increased security this can be disallowed using CREATE ROLE name SUPERUSER NOREPLICATION.
A role must be explicitly given permission to create databases (except for superusers, since those bypass all permission checks). To create such a role, use CREATE ROLE name CREATEDB.
A role must be explicitly given permission to create more roles (except for superusers, since those bypass all permission checks). To create such a role, use CREATE ROLE name CREATEROLE. A role with CREATEROLE privilege can alter and drop other roles, too, as well as grant or revoke membership in them. However, to create, alter, drop, or change membership of a superuser role, superuser status is required; CREATEROLE is insufficient for that.
A role must explicitly be given permission to initiate streaming replication. A role used for streaming replication must always have LOGIN permission as well. To create such a role, use CREATE ROLE name REPLICATION LOGIN.
A password is only significant if the client authentication method requires the user to supply a password when connecting to the database. The password and md5 authentication methods make use of passwords. Database passwords are separate from operating system passwords. Specify a password upon role creation with CREATE ROLE name PASSWORD 'string'.
A role's attributes can be modified after creation with ALTER ROLE. See the reference pages for the CREATE ROLE and ALTER ROLE commands for details.
Tip: It is good practice to create a role that has the CREATEDB and CREATEROLE privileges, but is not a superuser, and then use this role for all routine management of databases and roles. This approach avoids the dangers of operating as a superuser for tasks that do not really require it.
A role can also have role-specific defaults for many of the run-time configuration settings described in Chapter 17. For example, if for some reason you want to disable index scans (hint: not a good idea) anytime you connect, you can use:
ALTER ROLE myname SET enable_indexscan TO off;
This will save the setting (but not set it immediately). In subsequent connections by this role it will appear as though SET enable_indexscan TO off had been executed just before the session started. You can still alter this setting during the session; it will only be the default. To remove a role-specific default setting, use ALTER ROLE rolename RESET varname. Note that role-specific defaults attached to roles without LOGIN privilege are fairly useless, since they will never be invoked.