PERF: Join non unique

doc/source/whatsnew/v2.3.0.rst

@@ -101,6 +101,7 @@ Deprecations
 
 Performance improvements
 ~~~~~~~~~~~~~~~~~~~~~~~~
+- Performance improvement in :meth:`DataFrame.join` when left and/or right are non-unique and ``how`` is ``"left"``, ``"right"``, or ``"inner"`` (:issue:`56817`)
 - Performance improvement in :meth:`Index.take` when ``indices`` is a full range indexer from zero to length of index (:issue:`56806`)
 -
 

pandas/core/indexes/base.py

@@ -4809,11 +4809,18 @@ def _join_non_unique(
         left_idx, right_idx = get_join_indexers_non_unique(
             self._values, other._values, how=how, sort=sort
         )
-        mask = left_idx == -1
 
-        join_idx = self.take(left_idx)
-        right = other.take(right_idx)
-        join_index = join_idx.putmask(mask, right)
+        if how == "right":
+            join_index = other.take(right_idx)
+        else:
+            join_index = self.take(left_idx)
+
+        if how == "outer":
+            mask = left_idx == -1
+            if mask.any():
+                right = other.take(right_idx)
+                join_index = join_index.putmask(mask, right)
+
         if isinstance(join_index, ABCMultiIndex) and how == "outer":
             # test_join_index_levels
             join_index = join_index._sort_levels_monotonic()