Feature proposal and discussion: full-fledged column/function equivalence

Hi all;

I had a pleasant surprise today when demonstrating a previous misfeature in
PostgreSQL behaved unexpectedly. In further investigation, there is a
really interesting syntax which is very helpful for some things I had not
known about.

Consider the following:

CREATE TABLE keyvaltest (
key text primary key,
value text not null
);
INSERT INTO keyvaltest VALUES ('foo', 'bar'), ('fooprime', 'barprime');
SELECT value(k) from keyvaltest k;

The latter performs exactly like

SELECT k.value from keyvaltest k;

So the column/function equivalent is there. This is probably not the best
for production SQL code just because it is non-standard, but it is great
for theoretical examples because it shows the functional dependency between
tuple and tuple member.

It gets better:

CREATE OR REPLACE FUNCTION value(test) returns int language sql as $$
select 3; $$;
ERROR: "value" is already an attribute of type test

So this further suggests that value(test) is effectively an implicit
function of test (because it is a trivial functional dependency).

So with all this in mind, is there any reason why we can't or shouldn't
allow:

CREATE testfunction(test) returns int language sql as $$ select 1; $$;
SELECT testfunction FROM test;

That would allow first-class calculated columns.

I assume the work is mostly at the parser/grammatical level. Is there any
reason why supporting that would be a bad idea?
--
Best Wishes,
Chris Travers

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