Constant value for a partitioned table query inside a plpgsql function

When I explain a query using a partitioned table the result is the
expected. That is, only the corrected partition is scanned. But when the
query is inside a plpgsql function it takes forever to complete suggesting
it is scanning all partitions.

create table p (c integer);
create table p1 (like p);
alter table p1 add constraint p1c check (c = 1);
create table p2 (like p);
alter table p2 add constraint p2c check (c = 2);
insert into p1 values (1);
insert into p2 values (2);
alter table p1 inherit p;
alter table p2 inherit p;

The explain shows the expected plan and the select is also very fast:
(obviously the real query and table are more complex)

explain select c from p where c = 1;

A function like this takes very long to complete:

create or replace function pf() returns integer as
$body$
declare
v constant integer := 1;
begin
return (select c from p where c = v);
end
$body$
language plpgsql stable
cost 100;

Isn't the "constant" option to a variable declaration enough to the
planner? Or else what is the limitation here? Is there some way to see the
plan for a plpgsql function?

Regards, Clodoaldo