@dispatch(Reduction)
def _lean(expr, fields=None):
    child = expr._child
    try:
        fields = child.active_columns()
    except __HOLE__:
        fields = child.fields
    child, child_fields = _lean(child, fields=set(filter(None, fields)))
    return expr._subs({expr._child: child}), child_fields
dataset/ETHPy150Open blaze/blaze/blaze/expr/optimize.py/_lean