>>>>> "Tom" == Tom Lane <tgl@sss.pgh.pa.us> writes:
Tom> [ probably time to move this thread to -hackers ]
Tom> There is some moderately interesting reading material in sectionTom> 4.17.4 "Domain constraints" of SQL:2008. In
particular,itTom> appears to me that the standard goes out of its way to NOT claimTom> that every value that "is of" a
domaintype satisfies theTom> domain's constraints. It looks to me that the implementationTom> they have in mind is
thatdomain constraints are to be checked:
Tom> (1) when a value is assigned to a *table* column having thatTom> domain type;
Tom> (2) when a value is converted to that domain type by anTom> *explicit* cast construct;
Tom> (3) nowhere else.
By my reading it's a bit more involved than that. In particular, if
you cast from one rowtype to another, that seems to be defined in terms
of individual casts of each column, so CAST(ROW(null) TO rowtype) where
rowtype has one column of a not-null domain type would still count as an
explicit cast to the domain.
But there's a kicker: in Subclause 6.12, <cast specification>, in the
General Rules is:
2) Case: a) If the <cast operand> specifies NULL, then the result of CS is the null value and no further
GeneralRules of this Subclause are applied. b) If the <cast operand> specifies an <empty specification>, then
the result of CS is an empty collection of declared type TD and no further General Rules of this Subclause
areapplied. c) If SV is the null value, then the result of CS is the null value and no further General Rules
ofthis Subclause are applied.
That "no further General Rules" clause implies (assuming it's not a
blatant mistake in the spec) that this rule is therefore skipped in
the case of nulls:
21) If the <cast specification> contains a <domain name> and that <domain name> refers to a domain that contains a
<domain constraint> and if TV does not satisfy the <check constraint definition> simply contained in the <domain
constraint>,then an exception condition is raised: integrity constraint violation.
Which would imply that you can cast a NULL to a domain type even if
that would violate a constraint. Which would pretty much leave
actual assignment to storage as being the only place for the check
to happen.
--
Andrew (irc:RhodiumToad)
