Re: Range Types, constructors, and the type system

On Tue, 2011-06-28 at 10:58 -0400, Robert Haas wrote:
> On Mon, Jun 27, 2011 at 11:42 PM, Jeff Davis <pgsql(at)j-davis(dot)com> wrote:
> > So, in effect, RANGEINPUT is a special type used only for range
> > constructors. If someone tried to output it, it would throw an
> > exception, and we'd even have enough information at that point to print
> > a nice error message with a hint.
>
> I don't think I like the idea of throwing an error when you try to
> output it, but the rest seems reasonably sensible.

I thought it might add a little confusion if people thought they had a
range type but really had RANGEINPUT. For instance, if you do a "create
table as select range(1,2)" then the result might be slightly
unexpected.

But it's probably no more unexpected than "create table as select
'foo'". So, I suppose there's not much reason to throw an error. We can
just output it in the same format as a range type.

It's also much easier to explain something in the documentation that has
an output format, because at least it's tangible.

> > Actually, this is pretty much exactly Florian's idea (thanks again,
> > Florian), but at the time I didn't like it because "pair" didn't capture
> > everything that I wanted to capture, like infinite bounds, etc. But
> > there's no reason that it can't, and your point made me realize that --
> > you are effectively just using TEXT as the intermediate type (which
> > works, but has some undesirable characteristics).
>
> What undesirable characteristics?

Well, for one, outputting something as text and then reading it back in
does not always produce the same value. For instance, for float, it only
does that if you have extra_float_digits set to some high-enough value.
I suppose I could save the GUC, set it, and set it back; but that seems
like unnecessary ugliness.

There's also the deparsing/reparsing cycle. That might not really matter
for performance, but it seems unnecessary.

And there's always the fallback that "we have types for a reason".
Wouldn't it be odd if you wrote a query like:
select range(1,2) || 'foo'
and it succeeded? I'm sure that kind of thing can lead to some dangerous
situations.

Regards,
Jeff Davis