create table prices
(
  price_date date not null,   -- дата определения цены
  price      numeric(18, 4),  -- цена на указанную дату
  sells      integer,         -- число продаж
  volume     numeric(18, 4),  -- объем проданного товара
  moneys     numeric(18, 4),  -- стоимость проданного товара
  constraint pk_prices
    primary key (price_date)
);

UPDATE prices
SET price = prev.price, volume = 0, sells = 0, moneys = 0
FROM
  (SELECT * FROM prices) prev
WHERE prices.price_date = prev.price_date + INTERVAL '1 month'
  AND prices.volume = 0
  AND prices.price <> prev.price

FROM prices prev

UPDATE prices p
SET volume = 0, sells = 0, moneys = 0, price =
  (SELECT price FROM prices p2
    WHERE EXTRACT (DAY FROM p.price_date) =  EXTRACT (DAY FROM p2.price_date)
    AND p.price_date > p2.price_date
    AND (p2.volume > 0
         OR NOT EXISTS (SELECT * FROM prices p3 WHERE p3.price_date = p2.price_date - INTERVAL '1 month'))
   ORDER BY p2.price_date DESC
   LIMIT 1
  )
WHERE p.volume = 0

AND prices.price <> prev.price

UPDATE prices p
SET volume = 0, sells = 0, moneys = 0, price =
  (SELECT price FROM prices p2
    WHERE EXTRACT (DAY FROM p.price_date) =  EXTRACT (DAY FROM p2.price_date)
    AND p.price_date > p2.price_date
    AND (p2.volume > 0
         OR NOT EXISTS (SELECT * FROM prices p3 WHERE p3.price_date = p2.price_date - INTERVAL '1 month'))
   ORDER BY p2.price_date DESC
   LIMIT 1
  )
WHERE p.volume = 0 AND price <> 
  (SELECT price FROM prices p2
    WHERE EXTRACT (DAY FROM p.price_date) =  EXTRACT (DAY FROM p2.price_date)
    AND p.price_date > p2.price_date
    AND (p2.volume > 0
         OR NOT EXISTS (SELECT * FROM prices p3 WHERE p3.price_date = p2.price_date - INTERVAL '1 month'))
   ORDER BY p2.price_date DESC
   LIMIT 1
  )