[GitHub] [arrow] vibhatha commented on a diff in pull request #34651: GH-32763: [C++] Add FromProto for fetch & sort

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

vibhatha commented on code in PR #34651:
URL: https://github.com/apache/arrow/pull/34651#discussion_r1162239721


##########
cpp/src/arrow/engine/substrait/relation_internal.cc:
##########
@@ -367,6 +367,23 @@ ARROW_ENGINE_EXPORT Result<DeclarationInfo> MakeAggregateDeclaration(
 
 }  // namespace internal
 
+namespace {
+
+bool IsSortNullsFirst(const substrait::SortField::SortDirection& direction) {
+  return direction % 2 == 1;
+}
+
+compute::SortOrder SortOrderFromDirection(
+    const substrait::SortField::SortDirection& direction) {
+  if (direction < 3) {
+    return compute::SortOrder::Ascending;

Review Comment:
   Do we consider `UNSPECIFIED` as Ascending? 
   
   Here: 
   
   ```proto
    enum SortDirection {
       SORT_DIRECTION_UNSPECIFIED = 0;
       SORT_DIRECTION_ASC_NULLS_FIRST = 1;
       SORT_DIRECTION_ASC_NULLS_LAST = 2;
       SORT_DIRECTION_DESC_NULLS_FIRST = 3;
       SORT_DIRECTION_DESC_NULLS_LAST = 4;
       SORT_DIRECTION_CLUSTERED = 5;
     }
   ```
   
   I assume since the 1, 2 are Ascending, `<3` is to pick the first three as Ascending. 



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