Date: Wed, 21 Jul 2021 21:43:31 -0400
From: Bruce Momjian <bruce@momjian.us>
To: Bryn Llewellyn <bryn@yugabyte.com>
Cc: Tom Lane PostgreSQL <tgl@sss.pgh.pa.us>,
	Dean Rasheed <dean.a.rasheed@gmail.com>,
	Zhihong Yu <zyu@yugabyte.com>, Daniel Gustafsson <daniel@yesql.se>,
	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: <20210722014331.GB19630@momjian.us>
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On Wed, Jul 21, 2021 at 06:39:26PM -0700, Bryn Llewellyn wrote:
> Your statement
> 
> 
>     “months-to-days conversion is almost always an approximation, while the
>     days to seconds conversion is almost always accurate.” 
> 
> 
> is misleading. Any conversion like these (and also the “spill up” conversions
> that the justify_hours(), justify_days(), and justify_interval() built-in
> functions bring) are semantically dangerous because of the different rules for
> adding a pure months, a pure days, or a pure seconds interval to a timestamptz
> value.

We are trying to get the most reasonable output for fractional values
--- I stand by my statements.

> Unless you avoid mixed interval values, then it’s so hard (even though it is
> possible) to predict the outcomes of interval arithmetic. Rather, all you get
> is emergent behavior that I fail to see can be relied upon in deliberately
> designed application code. Here’s a telling example:

The point is that we will get unusual values, so we should do the best
we can.

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

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