The trick there is to ensure things work nicely if different subsets are iterated one after the other. Note that merely ordering isn't quite enough, let's say

The trick there is to ensure things work nicely if different subsets are iterated one after the other. Note that merely ordering isn't quite enough, let's say a particular system needs components ABC and component C is rare, then the ideal situation would be to sort A and B such that those entities with component C are close together.