Function new_join_children 

pub fn new_join_children(
    projection_as_columns: &[(Column, String)],
    far_right_left_col_ind: i32,
    far_left_right_col_ind: i32,
    left_child: &Arc<dyn ExecutionPlan>,
    right_child: &Arc<dyn ExecutionPlan>,
) -> Result<(ProjectionExec, ProjectionExec)>

If pushing down the projection over this join’s children seems possible, this function constructs the new ProjectionExecs that will come on top of the original children of the join.