Como obter o id da linha que foi selecionada pela função de agregação?

CREATE TEMP TABLE test (
    id integer, name text, amount numeric, datefrom timestamptz
);

COPY test FROM STDIN (FORMAT csv);
3,a,8,2018-01-01
4,a,3,2018-01-15 10:00
5,b,1,2018-02-20
6,b,1,2019-01-01
\.

Método 1. usando DISTINCT ON (específico do PostgreSQL)

SELECT DISTINCT ON (name)
  id, name, amount
FROM test
ORDER BY name, amount DESC, datefrom ASC;

Método 2. usando funções de janela

SELECT id, name, amount FROM (
  SELECT *, row_number() OVER (
    PARTITION BY name
    ORDER BY amount DESC, datefrom ASC) AS __rn
  FROM test) AS x
WHERE x.__rn = 1;

Método 3. usando subconsulta correlacionada

SELECT id, name, amount FROM test
WHERE id = (
  SELECT id FROM test AS t2
  WHERE t2.name = test.name
  ORDER BY amount DESC, datefrom ASC
  LIMIT 1
);