On Sun, Jun 30, 2024 at 3:56 AM Tom Lane <[email protected]> wrote: > Alvaro Herrera <[email protected]> writes: > >> + /* > >> + * For domains, consider the base type's typmod to decide whether to > >> setup > >> + * an implicit or explicit cast. > >> + */ > >> + if (get_typtype(returning->typid) == TYPTYPE_DOMAIN) > >> + (void) getBaseTypeAndTypmod(returning->typid, &baseTypmod); > > > TBH I'm not super clear on why we decide on explicit or implicit cast > > based on presence of a typmod. Why isn't it better to always use an > > implicit one? > > Hmm ... there are a bunch of existing places that seem to have similar > logic, but they are all in new-ish SQL/JSON functionality, and I would > not be surprised if they are all wrong. parse_coerce.c is quite > opinionated about what a domain's typtypmod means (see comments in > coerce_type() for instance); see also the logic in coerce_to_domain: > > * If the domain applies a typmod to its base type, build the appropriate > * coercion step. Mark it implicit for display purposes, because we don't > * want it shown separately by ruleutils.c; but the isExplicit flag passed > * to the conversion function depends on the manner in which the domain > * coercion is invoked, so that the semantics of implicit and explicit > * coercion differ. (Is that really the behavior we want?) > > I don't think that this SQL/JSON behavior quite matches that.
The reason I decided to go for the implicit cast only when there is a
typmod is that the behavior with COERCION_EXPLICIT is only problematic
when there's a typmod because of this code in
build_coercion_expression:
if (nargs == 3)
{
/* Pass it a boolean isExplicit parameter, too */
cons = makeConst(BOOLOID,
-1,
InvalidOid,
sizeof(bool),
BoolGetDatum(ccontext == COERCION_EXPLICIT),
false,
true);
args = lappend(args, cons);
}
Yeah, we could have fixed that by always using COERCION_IMPLICIT for
SQL/JSON but, as Jian said, we don't have a bunch of casts that these
SQL/JSON functions need, which is why I guess we ended up with
COERCION_EXPLICIT here in the first place.
One option I hadn't tried was using COERCION_ASSIGNMENT instead, which
seems to give coerceJsonFuncExpr() the casts it needs with the
behavior it wants, so how about applying the attached?
--
Thanks, Amit Langote
v6-0001-SQL-JSON-Rethink-c2d93c3802b.patch
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