hstore to json function from Ilya Kosmodemiansky

hs_2json.sql

CREATE FUNCTION public.hs_2json(hs hstore) RETURNS text AS $$
DECLARE
  rv text;
  r record;
BEGIN
  rv:='';
  for r in (select key, val from each(hs) as h(key, val)) loop
    if rv<>'' then
      rv:=rv||',';
    end if;
    rv:=rv || '"'  || r.key || '":';
    rv:=rv || '"' || regexp_replace(regexp_replace(r.val,$$\\$$,$$\\\\$$,'g'),'"', $$\"$$,'g') || '"';
  end loop;
  return '{'||rv||'}';
END;
$$ LANGUAGE plpgsql IMMUTABLE;

You missed the count in line 6 ;)