The implementation of semiJoin in EnumerableDefaults.java is based on two elements: an outer enumerator and an inner lookup. The method "returns elements of outer for which there is a member of inner with a matching key".
In order to achieve that, the innerLookup is always eagerly computed, even though in some cases it might be not necessary at all: when the outer enumerator returns no element there is no need for the innerLookup. In a worst case scenario, a time-consuming innerLookup computation combined with an empty outer enumerator will lead to an inefficient execution, which could have been easily avoided.
In order to improve that, it is proposed to delay the computation of the innerLookup until the moment when we are sure that it will be really needed, i.e. when the first outer enumerator item is processed.