The query mentioned by @janardhan shouldn’t be any slower. Note that DP has already been calculated. So, all we’re doing in Breakdown2 is to count the length of this DP variable, which is very cheap.
The issue with maintaining a count() along with count(uid) is complexity. It would be confusing to a user, when to use one and when to use another – we try to keep things as simple as possible in the language.