Fix struct row comparator's exception on empty structs (#10604)

Fixes #10603

This PR is to fix a bug of the optimized struct row comparator. For now,  the struct row comparator assumes that structs being compared are non-empty, so it fails when comparing empty structs.

Authors:
  - Alfred Xu (https://github.com/sperlingxx)
  - Devavret Makkar (https://github.com/devavret)

Approvers:
  - Devavret Makkar (https://github.com/devavret)
  - Nghia Truong (https://github.com/ttnghia)

URL: #10604

cpp/benchmarks/sort/sort_structs.cpp

@@ -80,5 +80,5 @@ void nvbench_sort_struct(nvbench::state& state)
 NVBENCH_BENCH(nvbench_sort_struct)
   .set_name("sort_struct")
   .add_int64_power_of_two_axis("NumRows", {10, 18, 26})
-  .add_int64_axis("Depth", {1, 8})
+  .add_int64_axis("Depth", {0, 1, 8})
   .add_int64_axis("Nulls", {0, 1});

cpp/include/cudf/table/experimental/row_operators.cuh

@@ -197,7 +197,11 @@ class device_row_comparator {
           return cuda::std::make_pair(state, depth);
         }
 
-        // Structs have been modified to only have 1 child when using this.
+        if (lcol.num_child_columns() == 0) {
+          return cuda::std::make_pair(weak_ordering::EQUIVALENT, depth);
+        }
+
+        // Non-empty structs have been modified to only have 1 child when using this.
         lcol = lcol.children()[0];
         rcol = rcol.children()[0];
         ++depth;

cpp/src/table/row_operators.cu

@@ -85,57 +85,44 @@ auto decompose_structs(table_view table,
     auto const& col = table.column(col_idx);
     if (is_nested(col.type())) {
       // convert and insert
-      std::vector<column_view> r_verticalized_columns;
-      std::vector<int> r_verticalized_col_depths;
-      std::vector<column_view> flattened;
-      std::vector<int> depths;
-      // TODO: Here I added a bogus leaf column at the beginning to help in the while loop below.
-      //       Refactor the while loop so that it can handle the last case.
-      flattened.push_back(make_empty_column(type_id::INT32)->view());
-      std::function<void(column_view const&, int)> recursive_child = [&](column_view const& c,
-                                                                         int depth) {
-        flattened.push_back(c);
-        depths.push_back(depth);
-        if (c.type().id() == type_id::STRUCT) {
-          for (int child_idx = 0; child_idx < c.num_children(); ++child_idx) {
-            auto scol = structs_column_view(c);
-            recursive_child(scol.get_sliced_child(child_idx), depth + 1);
+      std::vector<std::vector<column_view>> flattened;
+      std::function<void(column_view const&, std::vector<column_view>*, int)> recursive_child =
+        [&](column_view const& c, std::vector<column_view>* branch, int depth) {
+          branch->push_back(c);
+          if (c.type().id() == type_id::STRUCT) {
+            for (int child_idx = 0; child_idx < c.num_children(); ++child_idx) {
+              auto scol = structs_column_view(c);
+              if (child_idx > 0) {
+                verticalized_col_depths.push_back(depth + 1);
+                branch = &flattened.emplace_back();
+              }
+              recursive_child(scol.get_sliced_child(child_idx), branch, depth + 1);
+            }
           }
-        }
-      };
-      recursive_child(col, 0);
-      int curr_col_idx     = flattened.size() - 1;
-      column_view curr_col = flattened[curr_col_idx];
-      while (curr_col_idx > 0) {
-        auto const& prev_col = flattened[curr_col_idx - 1];
-        if (not is_nested(prev_col.type())) {
-          // We hit a column that's a leaf so seal this hierarchy
-          r_verticalized_columns.push_back(curr_col);
-          r_verticalized_col_depths.push_back(depths[curr_col_idx - 1]);
-          curr_col = prev_col;
-        } else {
-          curr_col = column_view(prev_col.type(),
-                                 prev_col.size(),
+        };
+      auto& branch = flattened.emplace_back();
+      verticalized_col_depths.push_back(0);
+      recursive_child(col, &branch, 0);
+
+      for (auto const& branch : flattened) {
+        column_view curr_col = branch.back();
+        for (auto it = branch.crbegin() + 1; it < branch.crend(); ++it) {
+          curr_col = column_view(it->type(),
+                                 it->size(),
                                  nullptr,
-                                 prev_col.null_mask(),
+                                 it->null_mask(),
                                  UNKNOWN_NULL_COUNT,
-                                 prev_col.offset(),
+                                 it->offset(),
                                  {curr_col});
         }
-        --curr_col_idx;
+        verticalized_columns.push_back(curr_col);
       }
-      verticalized_columns.insert(
-        verticalized_columns.end(), r_verticalized_columns.rbegin(), r_verticalized_columns.rend());
-      verticalized_col_depths.insert(verticalized_col_depths.end(),
-                                     r_verticalized_col_depths.rbegin(),
-                                     r_verticalized_col_depths.rend());
       if (not column_order.empty()) {
-        new_column_order.insert(
-          new_column_order.end(), r_verticalized_columns.size(), column_order[col_idx]);
+        new_column_order.insert(new_column_order.end(), flattened.size(), column_order[col_idx]);
       }
       if (not null_precedence.empty()) {
         new_null_precedence.insert(
-          new_null_precedence.end(), r_verticalized_columns.size(), null_precedence[col_idx]);
+          new_null_precedence.end(), flattened.size(), null_precedence[col_idx]);
       }
     } else {
       verticalized_columns.push_back(col);

cpp/tests/sort/sort_test.cpp

@@ -807,6 +807,60 @@ TYPED_TEST(FixedPointTestAllReps, FixedPointSortedOrderGather)
   CUDF_TEST_EXPECT_TABLES_EQUAL(sorted_table, sorted->view());
 }
 
+struct SortCornerTest : public BaseFixture {
+};
+
+TEST_F(SortCornerTest, WithEmptyStructColumn)
+{
+  using int_col = fixed_width_column_wrapper<int32_t>;
+
+  // struct{}, int, int
+  int_col col_for_mask{{0, 0, 0, 0, 0, 0}, {1, 0, 1, 1, 1, 1}};
+  auto null_mask  = cudf::copy_bitmask(col_for_mask.release()->view());
+  auto struct_col = cudf::make_structs_column(6, {}, UNKNOWN_NULL_COUNT, std::move(null_mask));
+
+  int_col col1{{1, 2, 3, 1, 2, 3}};
+  int_col col2{{1, 1, 1, 2, 2, 2}};
+  table_view input{{struct_col->view(), col1, col2}};
+
+  int_col expected{{1, 0, 3, 4, 2, 5}};
+  std::vector<order> column_order{order::ASCENDING, order::ASCENDING, order::ASCENDING};
+  auto got = sorted_order(input, column_order);
+  CUDF_TEST_EXPECT_COLUMNS_EQUAL(expected, got->view());
+
+  // struct{struct{}, int}
+  int_col col3{{0, 1, 2, 3, 4, 5}};
+  std::vector<std::unique_ptr<cudf::column>> child_columns;
+  child_columns.push_back(std::move(struct_col));
+  child_columns.push_back(col3.release());
+  auto struct_col2 =
+    cudf::make_structs_column(6, std::move(child_columns), 0, rmm::device_buffer{});
+  table_view input2{{struct_col2->view()}};
+
+  int_col expected2{{5, 4, 3, 2, 0, 1}};
+  auto got2 = sorted_order(input2, {order::DESCENDING});
+  CUDF_TEST_EXPECT_COLUMNS_EQUAL(expected2, got2->view());
+
+  // struct{struct{}, struct{int}}
+  int_col col_for_mask2{{0, 0, 0, 0, 0, 0}, {1, 0, 1, 1, 0, 1}};
+  auto null_mask2 = cudf::copy_bitmask(col_for_mask2.release()->view());
+  std::vector<std::unique_ptr<cudf::column>> child_columns2;
+  auto child_col_1 = cudf::make_structs_column(6, {}, UNKNOWN_NULL_COUNT, std::move(null_mask2));
+  child_columns2.push_back(std::move(child_col_1));
+  int_col col4{{5, 4, 3, 2, 1, 0}};
+  std::vector<std::unique_ptr<cudf::column>> grand_child;
+  grand_child.push_back(std::move(col4.release()));
+  auto child_col_2 = cudf::make_structs_column(6, std::move(grand_child), 0, rmm::device_buffer{});
+  child_columns2.push_back(std::move(child_col_2));
+  auto struct_col3 =
+    cudf::make_structs_column(6, std::move(child_columns2), 0, rmm::device_buffer{});
+  table_view input3{{struct_col3->view()}};
+
+  int_col expected3{{4, 1, 5, 3, 2, 0}};
+  auto got3 = sorted_order(input3, {order::ASCENDING});
+  CUDF_TEST_EXPECT_COLUMNS_EQUAL(expected3, got3->view());
+};
+
 }  // namespace test
 }  // namespace cudf