SET [ SESSION | LOCAL ] SESSION AUTHORIZATION username
SET [ SESSION | LOCAL ] SESSION AUTHORIZATION DEFAULT
RESET SESSION AUTHORIZATION   This command sets the session user identifier and the current user
   identifier of the current SQL-session context to be
   username.  The user name may be written as
   either an identifier or a string literal.
   The session user identifier is valid 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.
  
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.
The session user identifier may be changed only if the initial session user (the authenticated user) had the superuser privilege. Otherwise, the command is accepted only if it specifies the authenticated user name.
   The SESSION and LOCAL modifiers act the same
   as for the regular SET
   command.
  
The DEFAULT and RESET forms reset the session and current user identifiers to be the originally authenticated user name. These forms are always accepted.
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
   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.