From: Bruce Momjian <pgman@candle.pha.pa.us>
Message-Id: <200603012033.k21KXX125006@candle.pha.pa.us>
Subject: Re: [SQL] Interval subtracting
In-Reply-To: <14834.1141244891@sss.pgh.pa.us>
To: Tom Lane <tgl@sss.pgh.pa.us>
Date: Wed, 1 Mar 2006 15:33:33 -0500 (EST)
CC: Mark Dilger <pgsql@markdilger.com>, pgsql-hackers@postgresql.org
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit
Content-Type: text/plain; charset=US-ASCII

Tom Lane wrote:
> Bruce Momjian <pgman@candle.pha.pa.us> writes:
> > I don't think we can accept a change that takes a negative and turns it
> > into a positive and negative.
> 
> Yeah, I find the patch's changes to the regression results pretty
> disturbing.
> 
> Perhaps the correct definition ought to be like "if month part >= 0
> then the reduced day part should be between 0 and 30, otherwise the
> reduced day part should be between 0 and -30".  However there are
> still corner cases to worry about.  If the original month and day
> parts are of different sign, you might not be able to do such a
> reduction without changing the sign of the month part, consider
> "1 month -95 days".  Not clear what to do with this.
> 
> I guess I would expect a good result to satisfy one of these three
> cases:
> 	* month > 0 and 0 <= day < 30
> 	* month < 0 and -30 < day <= 0
> 	* month = 0 and -30 < day < 30
> If you believe that then "1 month -95 days" should justify to
> "-2 months -5 days".

I believe it.  :-)

-- 
  Bruce Momjian   http://candle.pha.pa.us
  SRA OSS, Inc.   http://www.sraoss.com

  + If your life is a hard drive, Christ can be your backup. +