DataObject optimization

Hi, Is it useful to delete dataobject (if we want more performance) with $do = null or unset($do) ? Is it depending on the instanciate method : factory or new ? What's the best way ? Does it cost a lot ? Another thing, here is a code example : $do = new DataObject_foo; while ($do->fetch()) { $do2 = new DataObject_foo2; $do2->find(); while ($do2->fetch()) { //... } } the code above works differently : $do = new DataObject_foo; //i instanciate before loops $do2 = new DataObject_foo2; while ($do->fetch()) { $do2->find(); while ($do2->fetch()) { //... } } any method with the $do2 object works only on the first cycle of the first loop but i don't understand why ? Thx ! JP Dumont