Re: Casting issues with domains

Kevin Grittner <kgrittn@ymail.com> writes:
> Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> As far as that goes, I think the OP was unhappy about the performance
>> of the information_schema views, which in our implementation do exactly
>> that so that the exposed types of the view columns conform to the SQL
>> standard, even though the underlying catalogs use PG-centric types.
>> 
>> I don't believe that that's the only reason why the performance of the
>> information_schema views tends to be sucky, but it's certainly a reason.

> Is that schema too "edge case" to justify some functional indexes
> on the cast values on the underlying catalogs? (I'm inclined to
> think so, but it seemed like a question worth putting out
> there....)

We don't support functional indexes on system catalogs, so whether it'd
be justified is sorta moot.  On the whole though I'm inclined to agree
that the information_schema views aren't used enough to justify adding
overhead to system-catalog updates, even if the pieces for that all
existed.

> Or, since these particular domains are known, is there any sane way
> to "special-case" these to allow the underlying types to work?

I don't particularly care for a kluge solution here.

I notice that recent versions of the SQL spec contain the notion of a
"distinct type", which is a user-defined type that is representationally
identical to some base type but has its own name, and comes equipped with
assignment-grade casts to and from the base type (which in PG terms would
be binary-compatible casts, though the spec doesn't require that).
It seems like this might be intended to be the sort of "zero cost type
alias" we were talking about, except that the SQL committee seems to have
got it wrong by not specifying the cast-to-base-type as being implicit.
Which ISTM you would want so that operators/functions on the base type
would apply automatically to the distinct type.  But perhaps we could
extend the spec with some option to CREATE TYPE to allow the cast to come
out that way.

Or in short, maybe we should try to replace the domains used in the
current information_schema with distinct types.
        regards, tom lane