This command sets the session user identifier and the current user identifier of the current SQL-session context to be username.
The session user identifier is initially set to be the (possibly authenticated) user name provided by the client. The current user identifier is normally equal to the session user identifier, but may change temporarily in the context of "setuid" functions and similar mechanisms. The current user identifier is relevant for permission checking.
Execution of this command is only permitted if the initial session user (the authenticated user) had the superuser privilege. This permission is kept for the duration of a connection; for example, it is possible to temporarily become an unprivileged user and later switch back to become a superuser.
SELECT SESSION_USER, CURRENT_USER; current_user | session_user --------------+-------------- peter | peter SET SESSION AUTHORIZATION 'paul'; SELECT SESSION_USER, CURRENT_USER; current_user | session_user --------------+-------------- paul | paul
SQL99 allows some other expressions to appear in place of the literal username which are not important in practice. PostgreSQL allows identifier syntax ("username"), which SQL does not. SQL does not allow this command during a transaction; PostgreSQL does not make this restriction because there is no reason to. The privileges necessary to execute this command are left implementation-defined by the standard.