PERF: join empty frame

doc/source/whatsnew/v1.5.0.rst

@@ -254,6 +254,7 @@ Performance improvements
 - Performance improvement in :meth:`.GroupBy.transform` when broadcasting values for user-defined functions (:issue:`45708`)
 - Performance improvement in :meth:`.GroupBy.transform` for user-defined functions when only a single group exists (:issue:`44977`)
 - Performance improvement in :func:`merge` when left and/or right are empty (:issue:`45838`)
+- Performance improvement in :meth:`DataFrame.join` when left and/or right are empty (:issue:`46015`)
 - Performance improvement in :class:`DataFrame` and :class:`Series` constructors for extension dtype scalars (:issue:`45854`)
 -
 

pandas/core/indexes/base.py

@@ -4544,12 +4544,22 @@ def join(
 
         if len(other) == 0 and how in ("left", "outer"):
             join_index = self._view()
-            rindexer = np.repeat(np.intp(-1), len(join_index))
+            rindexer = np.broadcast_to(np.intp(-1), len(join_index))
             return join_index, None, rindexer
 
         if len(self) == 0 and how in ("right", "outer"):
             join_index = other._view()
-            lindexer = np.repeat(np.intp(-1), len(join_index))
+            lindexer = np.broadcast_to(np.intp(-1), len(join_index))
+            return join_index, lindexer, None
+
+        if len(self) == 0 and how in ("left", "inner", "cross"):
+            join_index = self._view()
+            rindexer = np.array([])
+            return join_index, None, rindexer
+
+        if len(other) == 0 and how in ("right", "inner", "cross"):
+            join_index = other._view()
+            lindexer = np.array([])
             return join_index, lindexer, None
 
         if self._join_precedence < other._join_precedence:

pandas/tests/reshape/merge/test_join.py

@@ -881,3 +881,44 @@ def test_join_multiindex_not_alphabetical_categorical(categories, values):
         }
     ).set_index(["first", "second"])
     tm.assert_frame_equal(result, expected)
+
+
+@pytest.mark.parametrize(
+    "left_empty, how, exp",
+    [
+        (False, "left", "left"),
+        (False, "right", "empty"),
+        (False, "inner", "empty"),
+        (False, "outer", "left"),
+        (False, "cross", "empty"),
+        (True, "left", "empty"),
+        (True, "right", "right"),
+        (True, "inner", "empty"),
+        (True, "outer", "right"),
+        (True, "cross", "empty"),
+    ],
+)
+def test_join_empty(left_empty, how, exp):
+
+    left = DataFrame({"A": [2, 1], "B": [3, 4]}, dtype="int64").set_index("A")
+    right = DataFrame({"A": [1], "C": [5]}, dtype="int64").set_index("A")
+
+    if left_empty:
+        left = left.head(0)
+    else:
+        right = right.head(0)
+
+    result = left.join(right, how=how)
+
+    if exp == "left":
+        expected = DataFrame({"A": [2, 1], "B": [3, 4], "C": [np.nan, np.nan]})
+        expected = expected.set_index("A")
+    elif exp == "right":
+        expected = DataFrame({"B": [np.nan], "A": [1], "C": [5]})
+        expected = expected.set_index("A")
+    elif exp == "empty":
+        expected = DataFrame(index=Index([]), columns=["B", "C"], dtype="int64")
+        if how != "cross":
+            expected = expected.rename_axis("A")
+
+    tm.assert_frame_equal(result, expected)