Fix null check when comparing rows of structs in min and max redu…

…ction/groupby operations (#9994)

When comparing structs, we need to flatten its view into a `table_view` and compare the table's rows. Nulls check for the comparison needs to be done by checking nulls in the input structs column at all levels.

This PR fixes a bug that checks for nulls only at the top level. Unit tests designed specifically to detect this bug have also been added.

Authors:
  - Nghia Truong (https://github.com/ttnghia)

Approvers:
  - Mike Wilson (https://github.com/hyperbolic2346)
  - Bradley Dice (https://github.com/bdice)

URL: #9994

cpp/src/reductions/struct_minmax_util.cuh

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2021, NVIDIA CORPORATION.
+ * Copyright (c) 2022, NVIDIA CORPORATION.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -23,6 +23,7 @@
 #include <cudf/detail/utilities/vector_factories.hpp>
 #include <cudf/table/row_operators.cuh>
 #include <cudf/table/table_device_view.cuh>
+#include <cudf/table/table_view.hpp>
 
 namespace cudf {
 namespace reduction {
@@ -97,7 +98,7 @@ class comparison_binop_generator {
         table_view{{input}}, {}, std::vector<null_order>{DEFAULT_NULL_ORDER})},
       d_flattened_input_ptr{table_device_view::create(flattened_input, stream)},
       is_min_op(is_min_op),
-      has_nulls{input.has_nulls()},
+      has_nulls{has_nested_nulls(table_view{{input}})},
       null_orders_dvec(0, stream)
   {
     if (is_min_op) {

cpp/tests/groupby/max_tests.cpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2019-2021, NVIDIA CORPORATION.
+ * Copyright (c) 2019-2022, NVIDIA CORPORATION.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -388,5 +388,26 @@ TEST_F(groupby_max_struct_test, null_keys_and_values)
   test_single_agg(keys, vals, expect_keys, expect_vals, std::move(agg));
 }
 
+TEST_F(groupby_max_struct_test, values_with_null_child)
+{
+  constexpr int32_t null{0};
+  auto const keys = fixed_width_column_wrapper<int32_t>{1, 1};
+  auto const vals = [] {
+    auto child1 = fixed_width_column_wrapper<int32_t>{1, 1};
+    auto child2 = fixed_width_column_wrapper<int32_t>{{-1, null}, null_at(1)};
+    return structs_column_wrapper{child1, child2};
+  }();
+
+  auto const expect_keys = fixed_width_column_wrapper<int32_t>{1};
+  auto const expect_vals = [] {
+    auto child1 = fixed_width_column_wrapper<int32_t>{1};
+    auto child2 = fixed_width_column_wrapper<int32_t>{-1};
+    return structs_column_wrapper{child1, child2};
+  }();
+
+  auto agg = cudf::make_max_aggregation<groupby_aggregation>();
+  test_single_agg(keys, vals, expect_keys, expect_vals, std::move(agg));
+}
+
 }  // namespace test
 }  // namespace cudf

cpp/tests/groupby/min_tests.cpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2019-2021, NVIDIA CORPORATION.
+ * Copyright (c) 2019-2022, NVIDIA CORPORATION.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -387,5 +387,26 @@ TEST_F(groupby_min_struct_test, null_keys_and_values)
   test_single_agg(keys, vals, expect_keys, expect_vals, std::move(agg));
 }
 
+TEST_F(groupby_min_struct_test, values_with_null_child)
+{
+  constexpr int32_t null{0};
+  auto const keys = fixed_width_column_wrapper<int32_t>{1, 1};
+  auto const vals = [] {
+    auto child1 = fixed_width_column_wrapper<int32_t>{1, 1};
+    auto child2 = fixed_width_column_wrapper<int32_t>{{-1, null}, null_at(1)};
+    return structs_column_wrapper{child1, child2};
+  }();
+
+  auto const expect_keys = fixed_width_column_wrapper<int32_t>{1};
+  auto const expect_vals = [] {
+    auto child1 = fixed_width_column_wrapper<int32_t>{1};
+    auto child2 = fixed_width_column_wrapper<int32_t>{{null}, null_at(0)};
+    return structs_column_wrapper{child1, child2};
+  }();
+
+  auto agg = cudf::make_min_aggregation<groupby_aggregation>();
+  test_single_agg(keys, vals, expect_keys, expect_vals, std::move(agg));
+}
+
 }  // namespace test
 }  // namespace cudf