SELECT distinct on (prices.item_id) *
FROM prices
ORDER BY prices.item_id, prices.updated_at DESC

select sum(p.price)
from (select distinct on (p.item_id) p.*
      from prices p
      order by p.item_id, p.updated_at desc
     ) p

select distinct on (p.item_id) sum(prices.price) over ()
from prices p
order by p.item_id, p.updated_at desc

select sum(p.price)
from (select p.*,
             row_number() over (partition by p.item_id order by updated_at desc) as seqnum
      from prices p
      order by p.item_id, p.updated_at desc
     ) p
where seqnum = 1