UPDATE physical_person p  SET (request_count)=
  (SELECT k.request_count
     FROM (SELECT a.id_subject_union, COUNT(1) AS request_count
                 FROM physical_person  a
                 INNER JOIN request    r ON a.id_subject=r.id_subject
                   GROUP BY a.id_subject_union) k
       WHERE p.id_subject_union=k.id_subject_union
   ) ;