array_subtraction.sql

create or replace function diff_elements(anyarray, anyarray)
returns anyarray language sql immutable as $$
select array(
  select unnest($2)
  except
  select unnest($1)
);$$;

create operator - (
procedure = diff_elements,
leftarg=anyarray,
rightarg=anyarray
);

=# select array['a', 'n', 'z'] - array['n', 'z', 'd', 'e'];
?column? 
----------
{d,e}
(1 row)

=# select array[1, 5, 6] - array[5, 6, 23, 64];
?column? 
----------
{23,64}
(1 row)