Add unit test to cover default frames

Author: timsaucer

python/datafusion/tests/test_dataframe.py

@@ -468,6 +468,44 @@ def test_invalid_window_frame(units, start_bound, end_bound):
         WindowFrame(units, start_bound, end_bound)
 
 
+def test_window_frame_defaults_match_postgres(partitioned_df):
+    # ref: https://github.com/apache/datafusion-python/issues/688
+
+    window_frame = WindowFrame("rows", None, None)
+
+    col_a = column("a")
+
+    # Using `f.window` with or without an unbounded window_frame produces the same
+    # results. These tests are included as a regression check but can be removed when
+    # f.window() is deprecated in favor of using the .over() approach.
+    no_frame = f.window("avg", [col_a]).alias("no_frame")
+    with_frame = f.window("avg", [col_a], window_frame=window_frame).alias("with_frame")
+    df_1 = partitioned_df.select(col_a, no_frame, with_frame)
+
+    expected = {
+        "a": [0, 1, 2, 3, 4, 5, 6],
+        "no_frame": [3.0, 3.0, 3.0, 3.0, 3.0, 3.0, 3.0],
+        "with_frame": [3.0, 3.0, 3.0, 3.0, 3.0, 3.0, 3.0],
+    }
+
+    assert df_1.sort(col_a).to_pydict() == expected
+
+    # When order is not set, the default frame should be unounded preceeding to
+    # unbounded following. When order is set, the default frame is unbounded preceeding
+    # to current row.
+    no_order = f.avg(col_a).over(Window()).alias("over_no_order")
+    with_order = f.avg(col_a).over(Window(order_by=[col_a])).alias("over_with_order")
+    df_2 = partitioned_df.select(col_a, no_order, with_order)
+
+    expected = {
+        "a": [0, 1, 2, 3, 4, 5, 6],
+        "over_no_order": [3.0, 3.0, 3.0, 3.0, 3.0, 3.0, 3.0],
+        "over_with_order": [0.0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0],
+    }
+
+    assert df_2.sort(col_a).to_pydict() == expected
+
+
 def test_get_dataframe(tmp_path):
     ctx = SessionContext()