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Re: BUG #3431: age() gets the days wrong

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Pelle Johansson <pelle(at)morth(dot)org>
Cc: pgsql-bugs(at)postgresql(dot)org, pgsql-hackers(at)postgresql(dot)org
Subject: Re: BUG #3431: age() gets the days wrong
Date: 2007-07-08 21:56:26
Message-ID: 5675.1183931786@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-bugspgsql-hackerspgsql-patches
Pelle Johansson <pelle(at)morth(dot)org> writes:
> If you were to use the result for subtracting from the first value,  
> instead of adding to the second, the conditions are reversed. It's  
> not really as obvious as I first thought whether there's 2 months and  
> 29 days or 2 months and 30 days between 2006-11-02 and 2007-02-01...  

Hmm, that's a really good point; perhaps the original author was
thinking of it in those terms, in which case using the first month of
the interval is indeed sane.  (Almost: I believe that the loop can
iterate more than once, and then you need to look to the second month
etc.  The code's not doing that, so there's still a corner-case bug,
plus the fsec issue.)

Other than that corner case, it seems the behavior we currently have is
	if x > y, age() produces a positive interval such that
		x - age(x, y) = y
	if x < y, age() produces a negative interval such that
		y + age(x, y) = x

Are we satisfied with just documenting that, or do we want to change it,
and if so to what?

As the code currently stands, we have the symmetry property
	age(x,y) = - age(y,x)
for all x,y.  I don't think we can preserve that if we try to simplify
the relationship to interval addition/subtraction.

Comments?

			regards, tom lane

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