Range Types and extensions

I'd like to take another look at Range Types and whether part of it
should be an extension. Some of these issues relate to extensions in
general, not just range types.

First of all, what are the advantages to being in core?

1. ANYRANGE + CREATE TYPE ... AS RANGE
--------------------------------------
This is the most compelling, in my opinion. People can define new range
functions and new range types independently and each one gets the
benefit of the other automatically. Without this, there will be an
explosion of functions and a bunch of inconsistencies like functions
that support most range types but not all (merely because the function
author didn't know that the type existed).

In the several talks that I've given, a common question is related to
"multiranges" (ranges with holes). These get a little complex, and I
don't have a complete answer. However, multiranges can be approximated
with ordered arrays of non-overlapping, non-adjacent ranges. If someone
wants to take it upon themselves to develop a set of operators here,
that would be great -- but without ANYRANGE the operators would be
unmanageable.

2. Documentation and Tests
--------------------------
Let's say we take a minimalist view, and only have ANYRANGE and CREATE
TYPE ... AS RANGE in core; and leave the rest as an extension.

What exactly would the documentation say? I think it would be even more
hypothetical and abstract than the documentation for Exclusion
Constraints. So, there is a certain documentation advantage to having at
least enough functionality to allow someone to try out the feature.

And the tests for such a minimalist feature would be a significant
challenge -- what do we do there? Get pg_regress to load the extension
from PGXN?

3. Quality
----------
PostgreSQL has a great reputation for quality, and for good reason. But
extensions don't follow the same quality-control standards; and even if
some do, there is no visible stamp of approval. So, to ask someone to
use an extension means that they have to evaluate the quality for
themselves, which is a pretty high barrier.

Since PGXN (thanks David Wheeler) and EXTENSIONs (thanks Dmitri) solve
many of the other issues, quality control is one of the biggest ones
remaining. I still get questions about when the temporal type will be
"in core", and I think this is why.

I don't think this is a good excuse to put it in core though. We need to
solve this problem, and the best way to start is by getting
well-reviewed, high-quality extensions out there.

4. Future work -- RANGE KEY, RANGE FOREIGN KEY, RANGE MERGE JOIN, etc.
---------------------------------
There are a few aspects of range types that aren't in the first patch,
but are fairly obvious follow-up additions. These will require some
knowledge about ranges in the backend, like finding the "overlaps"
operator for a range. The current patch provides this knowledge by
providing a built-in overlaps operator for ANYRANGE. This would be a
non-issue if we had a good type interface system (that works on
polymorphic types) -- we could just have a built-in "range" interface,
and the range extension could add "&&" as the range interface's overlaps
operator for the type ANYRANGE.

=================================

So, where on this spectrum should range types fall? I think the most
minimalist would be to only support #1 (and the necessary type IO
functions); and leave all other functions, operators, and opclasses to
an extension. That has a lot of appeal, but I don't think we can ignore
the challenges above.

On the other hand, trying to make it a complete feature in core has
challenges as well. For instance, even with Range Types, Exclusion
Constraints aren't practical out-of-the-box unless we also have
BTree-GiST in core. So there's a snowball effect.

There might also be some middle ground, where its like the minimalist
approach, but with a few very basic constructors and accessors. That
would at least make it easier to test, but then to be actually useful
(with index support, operators, fancy functions, etc.) you'd need the
extension.

Thoughts?

Regards,
Jeff Davis