problem when using a while loop

[eluser]Spleshmen[/eluser]
i just want to learn how to use it with the while statement to, not just the foreach.
let me put it this way :
why this code its working

Code:

foreach($query->result() as $row) {
        $data[] = $row;
}

and this one is not

Code:

while($row = $query->result()) {
      $data[] = $row;
}
//if i use this code i get a error sayng :  
//Fatal error: Allowed memory size of 134217728 bytes exhausted (tried to allocate 35 bytes) in ...