Throwing mysqli result object into massive practice?

I was wondering if I have some code like:

$result = $db->query($sql); // dont worry, its escaped
$myData = (array)$result->fetch_assoc();  

      

        
        
        
      

    

Where

$result->fetch_assoc();

      

        
        
        
      

    

returns mysqli result object.

Is this exactly the case? I would assume that this is an expensive call - is it true?

It's pretty sure it's convenient to use right after the request for it, but I want to make sure I'm not doing anything bad.

Thanks for any input,