do $$
declare
  l_json json;
  l_json_str varchar;  
  rec    record;
begin
  l_json := json_build_array(
         json_build_object('c1',7,'c2',22.3), 
         json_build_object('c1',4'c2',2.9),
         json_build_object('c1',6,'c2',1.0));

  for rec in 
   (SELECT c1, c2 from table_name) 
  loop
    l_json_str := l_json_str || '{"c1" : '|| rec.c1 ||', "c2" : '|| rec.c2 ||'}';    
  end loop;
  l_json_str := '['||l_json_str||']';
  l_json := l_json_str;
end;
$$ language plpgsql;