I have applied the attached patch that documents the age() behavior,
plus fixes the mismatch sign for seconds by using part of Tom's earlier
patch.

I agree we want to keep the symmetry we have. We can call this item
closed.

---------------------------------------------------------------------------

Tom Lane wrote:
> Pelle Johansson <pelle@morth.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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--
Bruce Momjian <bruce@momjian.us> http://momjian.us
EnterpriseDB http://www.enterprisedb.com

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