Date: Tue, 19 Jun 2018 23:58:39 -0400
From: Bruce Momjian <bruce@momjian.us>
To: vodevsh@gmail.com, pgsql-docs@lists.postgresql.org
Subject: Re: DATE_PART('field', INTERVAL) not intuitive literal reading of
 interval
Message-ID: <20180620035839.GA32746@momjian.us>
References: <152698651482.26744.15456677499485530703@wrigleys.postgresql.org>
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On Tue, May 22, 2018 at 10:55:14AM +0000, PG Doc comments form wrote:
> The following documentation comment has been logged on the website:
> 
> Page: https://www.postgresql.org/docs/10/static/functions-datetime.html
> Description:
> 
> the result of
> t=# SELECT DATE_PART('month', '2018-05-31'::timestamp -
> '2018-02-24'::timestamp);
>  date_part
> -----------
>          0
> (1 row)
> 
> looks correct, but not intuitive, maybe we need a NB for:
> 
> t=# SELECT DATE_PART('month', justify_interval('2018-05-31'::timestamp -
> '2018-02-24'::timestamp));
>  date_part
> -----------
>          3
> (1 row)
> 
> https://stackoverflow.com/questions/50465632/postgresql-10-4-date-difference/50465676#50465676

This is really a function of how interval computes months, days, and
seconds from subtraction, which is outlined here:

	https://www.postgresql.org/docs/10/static/datatype-datetime.html#DATATYPE-INTERVAL-INPUT

I don't think adding something to the functions-datetime.html section
makes sense.  For example, this returns 1:

	SELECT extract(minutes from '1 hour 1 minute'::interval);
	 date_part
	-----------
        	 1

(Uh, it is kind of odd for EXTRACT to return a date_part column label.) 
The point is that the subtraction doesn't justify the values.

-- 
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

+ As you are, so once was I.  As I am, so you will be. +
+                      Ancient Roman grave inscription +