On 11/12/2010 07:07 PM, Jeff Davis wrote:
On Fri, 2010-11-12 at 08:34 -0800, David Fetter wrote:
Folks,
For the past couple of years, I've been hearing from the PostGIS
people among others that our type system just isn't flexible enough
for their needs. It's really starting to show its age, or possibly
design compromises that seemed reasonable a decade or more ago, but
are less so now.
To that end, I've put up a page on the wiki that includes a list of
issues to be addressed. It's intended to be changed, possibly
completely.
http://wiki.postgresql.org/wiki/Refactor_Type_System
What might the next version of the type system look like?
This problems (as stated) strikes me as pretty overwhelming.
1. As far as I can tell, we have the best type system of any SQL DBMS.
2. Getting a type system right is a hard problem by itself, and there
isn't any obvious consensus (although I think there is some agreement on
some issues).
3. Type systems are more challenging for a DBMS because you need to
account for things like storage, indexing, and optimization in ways that
programming languages don't (consider: when comparing an int4 and an
int8, you may want to coerce based on what indexes you have available).
4. SQL standard issues. In particular, I think that any modern type
system will run into pretty severe problems with NULLs in one way or
another. I think we'd have to pay very close attention to the standard
when designing a new type system, because I suspect that retrofitting
the standard onto a system we invent independently would be a disaster.
5. Backwards compatibility issues.
I think the best we'll do is be able to hack on some of the things that
we actively want and have clear use cases for, such as type interfaces.
We might have to give up on some of the more ambitious ideas that
involve propagating interesting information through the type inference
system; or having any real type that wasn't declared with CREATE TYPE.
Consider that right now we bundle the element type information along
with the array _value_.
Yeah, composites too, IIRC. It's a bit sad. But those are really just
warts show the difficulties we face in implementing types. I'm still
waiting for some seriously different yet possible thing we could do.
(And I agree we do have about the best type system around).
cheers
andrew
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