This defers O(N*S) operations, where
N = number of streams
S = number of subscribers per stream
In many cases we never do an O(N) operation on
a stream. Exceptions include:
- checking stream links from the compose box
- editing a stream
- adding members to a newly added stream
An operation that used to be O(N)--computing
the number of subscribers--is now O(1), and we
don't even pay O(N) on a one-time basis to
compute it (not counting the cost to build the
array from JSON, but we have to do that).
