constraint via selection

Dear list, 

I have two simplified tables "country" and "location":

create table country (code char(2) primary key,name varchar(60)
);

create table location (id serial primary key,country char(2) references country(code),[...]
);

Location should have a foreign key to country.code, but for performance & 
ressource reasons it would be much better to have a copy of the country 
code in location.country rather than a foreign key:

create table location (id serial primary key,country char(2) check ...
);

What would the most efficient formulation of the "check" constraint be in 
this case?

Horst