PERF: get_dummies create array of 0/1s with appropriate layout for internals (#40299)

Author: jorisvandenbossche

pandas/core/reshape/reshape.py

@@ -1039,7 +1039,8 @@ def get_empty_frame(data) -> DataFrame:
         return out
 
     else:
-        dummy_mat = np.eye(number_of_cols, dtype=dtype).take(codes, axis=0)
+        # take on axis=1 + transpose to ensure ndarray layout is column-major
+        dummy_mat = np.eye(number_of_cols, dtype=dtype).take(codes, axis=1).T
 
         if not dummy_na:
             # reset NaN GH4446