Re: Records, Types, and Arrays

On Fri, May 19, 2023 at 2:58 AM David G. Johnston <
david(dot)g(dot)johnston(at)gmail(dot)com> wrote:

> On Thursday, May 18, 2023, Raymond Brinzer <ray(dot)brinzer(at)gmail(dot)com> wrote:
>
>> scratch=# select row(2,3)::test_type;
>>
>
> Unknown typed value, immediately converted to a known concrete instance of
> test_type. It is never actually resolved as record.
>
> All of the others must concretely be resolved to record to escape their
> query level, and if you then try to cast the concrete record to some other
> concrete type a cast needs to exist.
>

I see. That suggests, oddly, that pg_typeof() is changing the thing it's
observing, because row(2,3) was not a record but an 'unknown typed value'
before pg_typeof() was called on it. Good to know.

CREATE CAST seems not to like working with pseudo-types. Neither the
source nor the target can be 'anyarray' or 'record'. So that seems out.

On the other hand, the pseudo-type doc says, 'Functions coded in C (whether
built-in or dynamically loaded) can be declared to accept or return any of
these pseudo data types.' I'm assuming that an 'unknown typed value"
corresponds to the 'unknown' pseudo-type. So it seems like a C function
which took an anyarray as its parameter, and returned a value of type
'unknown' would bypass the need for a specifically defined cast. (Emphasis
on "seems".)
--
Ray Brinzer