Re: Catalog domain not-null constraints

Vik Fearing <vik(at)postgresfriends(dot)org> writes:
> On 3/21/24 15:30, Tom Lane wrote:
>> The SQL spec's answer to that conundrum appears to be "NULL is
>> a valid value of every domain, and if you don't like it, tough".

> I don't see how you can infer this from the standard at all.

I believe where we got that from is 6.13 <cast specification>,
which quoth (general rule 2):

c) If SV is the null value, then the result of CS is the null
value and no further General Rules of this Subclause are applied.

In particular, that short-circuits application of the domain
constraints (GR 23), implying that CAST(NULL AS some_domain) is
always successful. Now you could argue that there's some other
context that would reject nulls, but being inconsistent with
CAST would seem more like a bug than a feature.

> As also said somewhere in that thread, I think that <cast specification>
> short-cutting a NULL input value without considering the constraints of
> a domain is a bug that needs to be fixed in the standard.

I think it's probably intentional. It certainly fits with the lack of
syntax for DOMAIN NOT NULL. Also, it's been like that since SQL99;
do you think nobody's noticed it for 25 years?

regards, tom lane