CLN use default_index more

doc/source/whatsnew/v2.0.0.rst

@@ -447,6 +447,7 @@ Performance improvements
 - Performance improvements to :func:`read_sas` (:issue:`47403`, :issue:`47405`, :issue:`47656`, :issue:`48502`)
 - Memory improvement in :meth:`RangeIndex.sort_values` (:issue:`48801`)
 - Performance improvement in :class:`DataFrameGroupBy` and :class:`SeriesGroupBy` when ``by`` is a categorical type and ``sort=False`` (:issue:`48976`)
+- Performance improvement in :func:`merge` when not merging on the index - the new index will now be :class:`RangeIndex` instead of :class:`Int64Index` (:issue:`49478`)
 
 .. ---------------------------------------------------------------------------
 .. _whatsnew_200.bug_fixes:

pandas/core/groupby/generic.py

@@ -95,6 +95,7 @@
     Index,
     MultiIndex,
     all_indexes_same,
+    default_index,
 )
 from pandas.core.indexes.category import CategoricalIndex
 from pandas.core.series import Series
@@ -1159,7 +1160,7 @@ def aggregate(self, func=None, *args, engine=None, engine_kwargs=None, **kwargs)
 
         if not self.as_index:
             self._insert_inaxis_grouper_inplace(result)
-            result.index = Index(range(len(result)))
+            result.index = default_index(len(result))
 
         return result
 
@@ -1778,7 +1779,7 @@ def nunique(self, dropna: bool = True) -> DataFrame:
         )
 
         if not self.as_index:
-            results.index = Index(range(len(results)))
+            results.index = default_index(len(results))
             self._insert_inaxis_grouper_inplace(results)
 
         return results

pandas/core/reshape/encoding.py

@@ -21,7 +21,10 @@
 from pandas.core.arrays import SparseArray
 from pandas.core.arrays.categorical import factorize_from_iterable
 from pandas.core.frame import DataFrame
-from pandas.core.indexes.api import Index
+from pandas.core.indexes.api import (
+    Index,
+    default_index,
+)
 from pandas.core.series import Series
 
 
@@ -249,7 +252,7 @@ def get_empty_frame(data) -> DataFrame:
         if isinstance(data, Series):
             index = data.index
         else:
-            index = Index(range(len(data)))
+            index = default_index(len(data))
         return DataFrame(index=index)
 
     # if all NaN

pandas/core/reshape/merge.py

@@ -84,6 +84,7 @@
 import pandas.core.common as com
 from pandas.core.construction import extract_array
 from pandas.core.frame import _merge_doc
+from pandas.core.indexes.api import default_index
 from pandas.core.sorting import is_int64_overflow_possible
 
 if TYPE_CHECKING:
@@ -1060,7 +1061,7 @@ def _get_join_info(
                 else:
                     join_index = self.left.index.take(left_indexer)
             else:
-                join_index = Index(np.arange(len(left_indexer)))
+                join_index = default_index(len(left_indexer))
 
         if len(join_index) == 0:
             join_index = join_index.astype(object)