Active Record: COUNT WHERE?

[eluser]BravoAlpha[/eluser]
[quote author="walesmd" date="1188990019"]

Code:

$count = $query->num_rows();

[/quote]
Well, I knew that one. I was wondering if there's any signifigant performance difference using this instead of a COUNT statement (or using my current method and counting the result with PHP). (I realize that any difference would probably not matter for me actual usage.)