Can anyone help me to understand why these are showing different outputs?

SELECT 
EXTRACT (epoch FROM ('2021-02-01 00:00:00'::timestamp - '2021-01-01 00:00:00'::timestamp)) / 3600 / 24 as time1,
EXTRACT (epoch FROM age('2021-02-01 00:00:00'::timestamp , '2021-01-01 00:00:00'::timestamp)) / 3600 / 24 as time2

output:

CodePudding user response：

There is a difference between the subtract operator and the function age(), per the documentation::

timestamp - timestamp › interval

Subtract timestamps (converting 24-hour intervals into days, similarly to justify_hours())

versus

age ( timestamp, timestamp ) › interval

Subtract arguments, producing a “symbolic” result that uses years and months, rather than just days

Example:

SELECT 
    '2021-02-01 00:00:00'::timestamp - '2021-01-01 00:00:00'::timestamp as interval1,
    age('2021-02-01 00:00:00'::timestamp , '2021-01-01 00:00:00'::timestamp) as interval2
    
 interval1 | interval2
----------- -----------
 31 days   | 1 mon
(1 row) 

The second interval is converted to 30 days in the further calculations described in the question.