Filter Subscription - Notify only when changes appeared

The subscription to a filter is a great feature. A possible improvement could be to send the email notification ONLY if the filter content has changed. A kind of "diff". It will avoid spam, and be more efficient for recipient.
I don't know how to do this today, so I guess it should be classified as an improvement request, ... if that feature is already available, please let me know how can I use it !

thanks in advance.

Thanks