Postgres 11

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

CREATE TABLE "values" (
    pk TEXT,
    i INTEGER,
    value REAL,
    PRIMARY KEY (pk, i)
);
insert into "values"
values
('A', 0, 17.5),
('A', 1, 32.7),
('A', 3, 5.3),
('B', 1, 13.5),
('B', 2, 4.8),
('B', 4, 89.1);
6 rows affected

with idx as (
  select pk, 
         generate_series(0, max(i)) as i
  from "values"
  group by pk
)
select idx.pk, 
       array_agg(v.value order by idx.i) as vals
from idx 
  left join "values" v on v.i = idx.i and v.pk = idx.pk
group by idx.pk  
;


pk
vals


A
{17.5,32.7,NULL,5.3}

B
{NULL,13.5,4.8,NULL,89.1}