pralabhkumar commented on a change in pull request #35191:
URL: https://github.com/apache/spark/pull/35191#discussion_r821299923



##########
File path: python/pyspark/pandas/series.py
##########
@@ -5228,9 +5229,20 @@ def asof(self, where: Union[Any, List]) -> Union[Scalar, 
"Series"]:
             where = [where]
         index_scol = self._internal.index_spark_columns[0]
         index_type = self._internal.spark_type_for(index_scol)
+
+        if np.nan in where:
+            # When `where` is np.nan, pandas returns the last index value.
+            last_index = 
self._internal.spark_frame.select(F.last(index_scol)).take(1)[0][0]
+            modified_where = [last_index if x is np.nan else x for x in where]
+        else:
+            modified_where = where
+
         cond = [
-            F.max(F.when(index_scol <= SF.lit(index).cast(index_type), 
self.spark.column))
-            for index in where
+            F.last(

Review comment:
       @ueshin  According to 
https://github.com/apache/spark/pull/35191#discussion_r799222433 . Last is 
being used , since assumption was dataframe is sorted on index_level_0 .  IF 
that assumption is not valid , then we cannot use Last . 
   
   cc @itholic 
   




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