[Backport #14551] PERF: casting loc to labels dtype before searchsorted (#14551)

(cherry picked from commit 1d95179)

Author: jorisvandenbossche

doc/source/whatsnew/v0.19.1.txt

@@ -24,7 +24,7 @@ Performance Improvements
 - Improved performance in ``.to_json()`` when ``lines=True`` (:issue:`14408`)
 - Improved performance in ``Series.asof(where)`` when ``where`` is a scalar (:issue:`14461)
 - Improved performance in ``DataFrame.asof(where)`` when ``where`` is a scalar (:issue:`14461)
-
+- Improved performance in certain types of `loc` indexing with a MultiIndex (:issue:`14551`).
 
 
 

pandas/indexes/multi.py

@@ -1907,6 +1907,13 @@ def convert_indexer(start, stop, step, indexer=indexer, labels=labels):
                 return np.array(labels == loc, dtype=bool)
             else:
                 # sorted, so can return slice object -> view
+                try:
+                    loc = labels.dtype.type(loc)
+                except TypeError:
+                    # this occurs when loc is a slice (partial string indexing)
+                    # but the TypeError raised by searchsorted in this case
+                    # is catched in Index._has_valid_type()
+                    pass
                 i = labels.searchsorted(loc, side='left')
                 j = labels.searchsorted(loc, side='right')
                 return slice(i, j)