Re: Range types

I wrote:
> The proposed problem is certainly soluble without any assumptions
> of discreteness.

To be concrete, I think it could be approached like this:

Assume the datatype provides a built-in function

period_except(p1 period, p2 period) returns setof period

which can return zero, one, or two rows depending on the inputs:

no rows if p1 is completely contained in p2

one row if p1 partially overlaps p2, for example:

[1,4] except [3,5] returns [1,3)
[4,6] except [1,5) returns [5,6]

two rows if p1 properly contains p2, for example

[1,10] except [4,5] returns [1,4) and (5,10]
[1,10] except [9,10) returns [1,9) and [10,10]

and of course just p1 if p1 and p2 don't overlap at all.

Given such a function it's a simple matter of successively removing each
element of p2[] from the set representing the current members of p1[].
The way that I'd find most natural to code that is a loop, along the
lines of

foreach p2_member in unnest(p2) loop
p1 := array(select period_except(p1_member, p2_member)
from unnest(p1) p1_member);
end loop;

But maybe it can be done in a single SQL command.

As this example makes clear, when dealing with continuous intervals you
*must* admit both open and closed intervals, else you don't have a way
to represent the results of "except". Maybe part of the failure to
communicate here arises from your desire to try to avoid supporting both
kinds of intervals. But I think you really have to do it if you want to
deal with data that hasn't got any natural granularity.

regards, tom lane