FIX-#6935: Fix Merge failed when right operand is an empty dataframe

modin/core/dataframe/pandas/dataframe/dataframe.py

@@ -3348,10 +3348,21 @@ def broadcast_apply_full_axis(
         PandasDataframe
             New Modin DataFrame.
         """
+
+        def get_partitions(df):
+            """Deal with the corner case if the "other" dataframe has no partitions."""
+            if df._partitions.size > 0:
+                return df._partitions
+            else:
+                empty_partition = self._partition_mgr_cls.create_partition_from_data(
+                    pandas.DataFrame(index=df.index, columns=df.columns)
+                )
+                return empty_partition
+
         if other is not None:
             if not isinstance(other, list):
                 other = [other]
-            other = [o._partitions for o in other] if len(other) else None
+            other = [get_partitions(o) for o in other] if len(other) else None
 
         if apply_indices is not None:
             numeric_indices = self.get_axis(axis ^ 1).get_indexer_for(apply_indices)

modin/core/dataframe/pandas/partitioning/partition_manager.py

@@ -175,6 +175,23 @@ def preprocess_func(cls, map_func):
 
     # END Abstract Methods
 
+    @classmethod
+    def create_partition_from_data(cls, data):
+        """
+        Create NumPy array of partitions that wrapps the given data.
+
+        Parameters
+        ----------
+        data : pandas.DataFrame or pandas.Series
+            Data that has to be wrapped in partition.
+
+        Returns
+        -------
+        np.ndarray
+            A NumPy 2D array of a single partition which contains the data.
+        """
+        return np.array([[cls._partition_class.put(data)]])
+
     @classmethod
     def column_partitions(cls, partitions, full_axis=True):
         """
@@ -1120,6 +1137,9 @@ def to_pandas_remote(df, partition_shape, *dfs):
                 (df,) + dfs, partition_shape, called_from_remote=True
             )
 
+        if partitions.size <= 1:
+            return partitions
+
         preprocessed_func = cls.preprocess_func(to_pandas_remote)
         partition_shape = partitions.shape
         partitions_flattened = partitions.flatten()

modin/pandas/test/dataframe/test_join_sort.py

@@ -383,6 +383,20 @@ def test_merge(test_data, test_data2):
         modin_df.merge("Non-valid type")
 
 
+def test_merge_empty():
+    data = np.random.uniform(0, 100, size=(2**6, 2**6))
+    pandas_df = pandas.DataFrame(data)
+    pandas_df2 = pandas_df.iloc[:0]
+    modin_df = pd.DataFrame(data)
+    modin_df_2 = modin_df.iloc[:0]
+    modin_result = pd.merge(
+        modin_df,
+        modin_df_2,
+    )
+    pandas_result = pandas.merge(pandas_df, pandas_df2)
+    df_equals(modin_result, pandas_result)
+
+
 def test_merge_with_mi_columns():
     modin_df1, pandas_df1 = create_test_dfs(
         {