Fix reductions when DataFrame has MulitIndex columns (#15097)

closes #15085

Authors:
  - Matthew Roeschke (https://github.com/mroeschke)

Approvers:
  - GALI PREM SAGAR (https://github.com/galipremsagar)
  - Bradley Dice (https://github.com/bdice)

URL: #15097

python/cudf/cudf/core/dataframe.py

@@ -3955,7 +3955,6 @@ def transpose(self):
             Not supporting *copy* because default and only behavior is
             copy=True
         """
-
         index = self._data.to_pandas_index()
         columns = self.index.copy(deep=False)
         if self._num_columns == 0 or self._num_rows == 0:
@@ -6202,9 +6201,13 @@ def _reduce(
                         "Columns must all have the same dtype to "
                         f"perform {op=} with {axis=}"
                     )
-                return Series._from_data(
-                    {None: as_column(result)}, as_index(source._data.names)
-                )
+                if source._data.multiindex:
+                    idx = MultiIndex.from_tuples(
+                        source._data.names, names=source._data.level_names
+                    )
+                else:
+                    idx = as_index(source._data.names)
+                return Series._from_data({None: as_column(result)}, idx)
         elif axis == 1:
             return source._apply_cupy_method_axis_1(op, **kwargs)
         else:

python/cudf/cudf/tests/test_reductions.py

@@ -366,3 +366,13 @@ def test_reductions_axis_none_warning(op):
     ):
         expected = getattr(pdf, op)(axis=None)
     assert_eq(expected, actual, check_dtype=False)
+
+
+def test_reduction_column_multiindex():
+    idx = cudf.MultiIndex.from_tuples(
+        [("a", 1), ("a", 2)], names=["foo", "bar"]
+    )
+    df = cudf.DataFrame(np.array([[1, 3], [2, 4]]), columns=idx)
+    result = df.mean()
+    expected = df.to_pandas().mean()
+    assert_eq(result, expected)