The idea is to have a $match that can be vectorized, followed by a $addFields that creates an array using scalar processing, followed by an $unwind that works on that scalar array, followed by another stage that can be vectorized (a $match that compares a block variables with the scalar value generated by the $addFields). The purpose of the change would be to let the $unwind propagate the "can produce vectorized" flag down to the child stage, then create the scalar operation and don't insert the final block_to_row