mengxr commented on a change in pull request #24897: [SPARK-28056] [PYTHON] add 
doc for SCALAR_ITER Pandas UDF
URL: https://github.com/apache/spark/pull/24897#discussion_r294596371
 
 

 ##########
 File path: docs/sql-pyspark-pandas-with-arrow.md
 ##########
 @@ -86,6 +86,22 @@ The following example shows how to create a scalar Pandas 
UDF that computes the
 </div>
 </div>
 
+### Scalar Iterator
+
+Scalar iterator (`SCALAR_ITER`) Pandas UDF is the same as scalar Pandas UDF 
above except that the
+underlying Python function takes an iterator of batches as input instead of a 
single batch and 
+it yields output batches instead of returning a single output batch.
+It is useful when the UDF execution requires initializing some states, e.g., 
loading an machine
+learning model file to apply inference to every input batch.
+
 
 Review comment:
   Updated.

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