Re: pgsql: Add more SQL/JSON constructor functions

Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org> 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.

While I'm bitching ... this coding style is bogus anyway:

>> + if (get_typtype(returning->typid) == TYPTYPE_DOMAIN)
>> + (void) getBaseTypeAndTypmod(returning->typid, &baseTypmod);

because it results in two syscache lookups not one. You are supposed
to apply getBaseTypeAndTypmod unconditionally, as is done everywhere
except in the SQL/JSON logic. I am also wondering how it can possibly
be sensible to throw away the function result of getBaseTypeAndTypmod
in this context.

regards, tom lane