[GitHub] [arrow] mrkn commented on a change in pull request #8612: ARROW-8199: [C++] Add support for multi-column sort indices on Table

[GitBox <gi...@apache.org>]

mrkn commented on a change in pull request #8612:
URL: https://github.com/apache/arrow/pull/8612#discussion_r519569356



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File path: cpp/src/arrow/compute/kernels/vector_sort.cc
##########
@@ -125,35 +128,43 @@ class CompareSorter {
           std::stable_partition(indices_begin, indices_end,
                                 [&values](uint64_t ind) { return !values.IsNull(ind); });
     }
-    std::stable_sort(indices_begin, nulls_begin,
-                     [&values](uint64_t left, uint64_t right) {
-                       return values.GetView(left) < values.GetView(right);
-                     });
+    if (options.order == SortOrder::ASCENDING) {
+      std::stable_sort(indices_begin, nulls_begin,
+                       [&values](uint64_t left, uint64_t right) {
+                         return values.GetView(left) < values.GetView(right);
+                       });
+    } else {
+      std::stable_sort(indices_begin, nulls_begin,
+                       [&values](uint64_t left, uint64_t right) {
+                         return values.GetView(left) > values.GetView(right);

Review comment:
       This requires that all of `==`, `<`, and `>` are implemented in the value type.
   Instead, writing `right < left` here only requires `==` and `<`.
   IMHO, requiring only `==` and `<` is better to support extension types in the future.




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