Date: Tue, 20 Jul 2021 22:48:34 -0400
From: Bruce Momjian <bruce@momjian.us>
To: Zhihong Yu <zyu@yugabyte.com>
Cc: Daniel Gustafsson <daniel@yesql.se>, Bryn Llewellyn <bryn@yugabyte.com>,
	Tom Lane PostgreSQL <tgl@sss.pgh.pa.us>,
	Justin Pryzby <pryzby@telsasoft.com>,
	Isaac Morland <isaac.morland@gmail.com>,
	John W Higgins <wishdev@gmail.com>,
	PostgreSQL-development <pgsql-hackers@postgresql.org>
Subject: Re: Have I found an interval arithmetic bug?
Message-ID: <20210721024834.GA8395@momjian.us>
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On Tue, Jul 20, 2021 at 05:13:37PM -0700, Zhihong Yu wrote:
> On Tue, Jul 20, 2021 at 3:53 PM Bruce Momjian <bruce@momjian.us> wrote:
> With your patch, the following example (Coutesy Bryn) still shows fraction:
> 
> # select (interval '1 month')*1.123;
>        ?column?
> -----------------------
>  1 mon 3 days 16:33:36
> (1 row) 
> 
> Do you think the output can be improved (by getting rid of fraction) ?

Well, I focused on how fractional units were processed inside of
interval values.  I never considered how multiplication should be
handled.  I have not really thought about how to handle that, but this
example now gives me concern:

	SELECT INTERVAL '1.06 months 1 hour';
	       interval
	-----------------------
	 1 mon 2 days 01:00:00

Notice that it rounds the '1.06 months' to '1 mon 2 days', rather than
spilling to hours/minutes/seconds, even though hours is already
specified.  I don't see a better way to handle this than the current
code already does, but it is something odd.

-- 
  Bruce Momjian  <bruce@momjian.us>        https://momjian.us
  EDB                                      https://enterprisedb.com

  If only the physical world exists, free will is an illusion.