TableOID in description of inlined function

Hi All,

If I have a table

create table my_table(id int primary key);

and a function

create function my_function() returns table(id int) stable as $$ select *
from my_table $$ language sql;

then Postgres knows that selecting from the function is really just
selecting from the table:

explain select * from my_function();
QUERY PLAN
------------------------------------------------------------
Seq Scan on my_table (cost=0.00..35.50 rows=2550 width=4)
(1 row)

But if you prepare the same select statement and ask Postgres for a
description of it, then in the response the column "id" will have a
TableOID of 0 - even though we know, on some level, that it's going to be
selected straight from a table. I started looking at this because sqlc (
https://github.com/sqlc-dev/sqlc) uses TableOID to infer the nullability of
the column.

Can someone explain how these two things are connected (or aren't)? Perhaps
more importantly, is there a workaround?