Answers for "how to compute difference of date part in postgresql"

postgresql difference between two dates in days

SELECT
  AGE('2012-03-05', '2010-04-01'),
  DATE_PART('year', AGE('2012-03-05', '2010-04-01')) AS years,
  DATE_PART('month', AGE('2012-03-05', '2010-04-01')) AS months,
  DATE_PART('day', AGE('2012-03-05', '2010-04-01')) AS days;
 
...  
-- This will give you full years, month, days ... between two dates:

--          age          | years | months | days
-- -----------------------+-------+--------+------
--  1 year 11 mons 4 days |     1 |     11 |    4

postgresql get difference in hours between two dates

select EXTRACT(
	EPOCH FROM now() - (now() - INTERVAL '5 HOUR')
)/3600

-- it will return 5