Tom Lane wrote:
>> - Can domains have operators, or are operators defined on types?
>
> I think the current state of play is that you can have such things but
> the system will only consider them for exact type matches, so you might
> need more explicit casts than you ordinarily would.
Turns out it's even smarter than that; it seems to coerce when it's unambiguous:
create domain birthdate as date;
create function date_dist(birthdate, birthdate) returns integer as $$ select 123;
$$ language sql;
create operator <-> (procedure = date_dist,leftarg = birthdate,rightarg = birthdate);
select '2012-01-01'::birthdate <-> '2012-01-01'::birthdate;
-- 123
select '2012-01-01'::date <-> '2012-01-01'::date ;
-- 123
create domain activity_date as date;
create function date_dist(activity_date, activity_date)
returns integer as $$ select 432;
$$ language sql;
create operator <-> (procedure = date_dist,leftarg = activity_date,rightarg = activity_date);
select '2012-01-01'::activity_date <-> '2012-01-01'::activity_date;
-- 432
select '2012-01-01'::birthdate <-> '2012-01-01'::birthdate;
-- 123
select '2012-01-01'::date <-> '2012-01-01'::date ;
-- ERROR: operator is not unique: date <-> date
