[GitHub] [spark] zhengruifeng commented on a diff in pull request #42798: [SPARK-43295][PS] Support string type columns for `DataFrameGroupBy.sum`

["zhengruifeng (via GitHub)" <gi...@apache.org>]

zhengruifeng commented on code in PR #42798:
URL: https://github.com/apache/spark/pull/42798#discussion_r1315417026


##########
python/pyspark/pandas/groupby.py:
##########
@@ -3534,7 +3534,12 @@ def _reduce_for_stat_function(
             for label in psdf._internal.column_labels:
                 psser = psdf._psser_for(label)
                 input_scol = psser._dtype_op.nan_to_null(psser).spark.column
-                output_scol = sfun(input_scol)
+                if sfun.__name__ == "sum" and isinstance(
+                    psdf._internal.spark_type_for(label), StringType
+                ):
+                    output_scol = F.concat_ws("", F.collect_list(input_scol))

Review Comment:
   since string columns are computed together with numerical ones, I think we have to compute strings' sum in an aggregation way:
   
   ```
   F.concat_ws("", F.array_sort(
     F.collect_list(F.struct(NATURAL_ORDER_COLUMN_NAME, input_scol))
   )
   ```
   
   For struct type, array_sort sort elements by first field then second field, IIRC



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