seleziona i valori massimi e minimi ogni x quantità di righe-postgresql

Puoi aggregare numeri fissi di righe usando row_number() e aritmetica:

select min(date),
       (array_agg(bid order by seqnum asc))[1] as open,
       (array_agg(bid order by seqnum desc))[1] as close,
       min(bid) as min_bid, max(bid) as max_bid
from (select t.*, row_number() over (order by date) as seqnum
      from ticks t
     ) t
group by floor((seqnum - 1) / 500);

Questo usa "hack" per ottenere l'open e close -- usando gli array.