Re: Have I found an interval arithmetic bug?

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: Bryn Llewellyn <bryn(at)yugabyte(dot)com>
Cc: Tom Lane PostgreSQL <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Dean Rasheed <dean(dot)a(dot)rasheed(at)gmail(dot)com>, Zhihong Yu <zyu(at)yugabyte(dot)com>, Daniel Gustafsson <daniel(at)yesql(dot)se>, Justin Pryzby <pryzby(at)telsasoft(dot)com>, Isaac Morland <isaac(dot)morland(at)gmail(dot)com>, John W Higgins <wishdev(at)gmail(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Have I found an interval arithmetic bug?
Date: 2021-07-22 01:43:31
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(at)momjian(dot)us> https://momjian.us
EDB https://enterprisedb.com

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

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