PERF: avoid copy in concatenate_array_managers if reindex already copies (#44559)

Author: jorisvandenbossche

pandas/core/internals/array_manager.py

@@ -527,8 +527,8 @@ def copy_func(ax):
         if deep:
             new_arrays = [arr.copy() for arr in self.arrays]
         else:
-            new_arrays = self.arrays
-        return type(self)(new_arrays, new_axes)
+            new_arrays = list(self.arrays)
+        return type(self)(new_arrays, new_axes, verify_integrity=False)
 
     def reindex_indexer(
         self: T,

pandas/core/internals/concat.py

@@ -77,10 +77,16 @@ def _concatenate_array_managers(
     # reindex all arrays
     mgrs = []
     for mgr, indexers in mgrs_indexers:
+        axis1_made_copy = False
         for ax, indexer in indexers.items():
             mgr = mgr.reindex_indexer(
                 axes[ax], indexer, axis=ax, allow_dups=True, use_na_proxy=True
             )
+            if ax == 1 and indexer is not None:
+                axis1_made_copy = True
+        if copy and concat_axis == 0 and not axis1_made_copy:
+            # for concat_axis 1 we will always get a copy through concat_arrays
+            mgr = mgr.copy()
         mgrs.append(mgr)
 
     if concat_axis == 1:
@@ -94,8 +100,6 @@ def _concatenate_array_managers(
         # concatting along the columns -> combine reindexed arrays in a single manager
         assert concat_axis == 0
         arrays = list(itertools.chain.from_iterable([mgr.arrays for mgr in mgrs]))
-        if copy:
-            arrays = [x.copy() for x in arrays]
 
     new_mgr = ArrayManager(arrays, [axes[1], axes[0]], verify_integrity=False)
     return new_mgr