perf(postprocessing): improve pivot postprocessing operation (#23465)

Co-authored-by: Usiel Riedl <usiel.riedl@automattic.com>

superset/utils/pandas_postprocessing/pivot.py

@@ -87,7 +87,7 @@ def pivot(  # pylint: disable=too-many-arguments,too-many-locals
     if not drop_missing_columns and columns:
         for row in df[columns].itertuples():
             for metric in aggfunc.keys():
-                series_set.add(str(tuple([metric]) + tuple(row[1:])))
+                series_set.add(tuple([metric]) + tuple(row[1:]))
 
     df = df.pivot_table(
         values=aggfunc.keys(),
@@ -101,10 +101,7 @@ def pivot(  # pylint: disable=too-many-arguments,too-many-locals
     )
 
     if not drop_missing_columns and len(series_set) > 0 and not df.empty:
-        for col in df.columns:
-            series = str(col)
-            if series not in series_set:
-                df = df.drop(col, axis=PandasAxis.COLUMN)
+        df = df.drop(df.columns.difference(series_set), axis=PandasAxis.COLUMN)
 
     if combine_value_with_metric:
         df = df.stack(0).unstack()