Tom Lane wrote:
>> We can ask the user to provide a prior() and next() function, and if
>> they aren't provided, we assume it's continuous.
>>
>
> Well, that still leaves us with the problem that Joe Schmo will file
> a bug when "create function next(float4) returns float4 as
> $$ select $1 + 0.00001 $$" doesn't behave sanely for him. I'd prefer
> not to leave it to the user to decide whether a type is discrete or
> not. The traffic on pgsql-bugs is convincing evidence that a very
> large fraction of our user-base doesn't understand that floats are
> inexact :-(
>
Indeed.
>
>> I think "countable" is a more accurate word than "discrete". Strings are
>> discrete but not countable.
>>
>
> It's been too long since college math classes for me to be sure whether
> "discrete" is really the exact term here. But I'm even more suspicious
> of "countable". I think a suitable diagonalization argument might show
> that strings are countable. That's getting a bit off-topic though...
>
>
>
Right, I don't think strings are any more or less countable than
integers. (and yes, it's a bit OT).
Surely the issue from our POV is whether, given two distinct members of
a class, we can ever say there is not any intervening member of the
class according to some ordering. If we can't then next() and prior()
make no sense for that class.
cheers
andrew