CREATE FUNCTION posts_added_max()
  RETURNS TRIGGER
  LANGUAGE plpgsql
  AS $$
BEGIN
  INSERT INTO posts_added_max AS pam("user", service, added)
  SELECT
    "user",
    service,
    max(added) AS added
  FROM
    posts
  WHERE
    posts.service = NEW.service
    AND posts."user" = NEW."user"
  GROUP BY
    "user",
    service
  ON CONFLICT(service,
    "user")
    DO UPDATE SET
      added = EXCLUDED.added
    WHERE
      EXCLUDED.added > pam.added;
  RETURN NULL;
END;
$$;