EnumerableDefaults#MergeJoinEnumerator provides an Enumerator that performs a merge join between two sorted inputs. This sort operation can be potentially very expensive, so we should skip it if possible. Right now, merge join inputs' enumerators are created when MergeJoinEnumerator is constructed; however, there are some cases where we can skip the enumerator creation of one input: if the outer (i.e. left) enumerator returns no results, there is no need to access (and sort) the inner (i.e. right) enumerator. For this reason, we should delay the inner enumerator creation until the moment we are sure it is really required: when the first element of the outer enumerator is fetched. This strategy is already in place in other join algorithms in EnumerableDefaults (e.g. nestedLoopJoinOptimized, semiEquiJoin), and it will be quite easy to apply on MergeJoinEnumerator.