Last-minute improvements:

- avoid copying type codes array
- remove skipping of sliced tests on dense unions
- add union-take test with null indices

cpp/src/arrow/compute/kernels/vector_selection_internal.cc

@@ -750,13 +750,14 @@ struct SparseUnionSelectionImpl
   LIFT_BASE_MEMBERS();
 
   TypedBufferBuilder<int8_t> child_id_buffer_builder_;
-  std::vector<int8_t> type_codes_;
+  const int8_t type_code_for_null_;
 
   SparseUnionSelectionImpl(KernelContext* ctx, const ExecSpan& batch,
                            int64_t output_length, ExecResult* out)
       : Base(ctx, batch, output_length, out),
         child_id_buffer_builder_(ctx->memory_pool()),
-        type_codes_(checked_cast<const UnionType&>(*this->values.type).type_codes()) {}
+        type_code_for_null_(
+            checked_cast<const UnionType&>(*this->values.type).type_codes()[0]) {}
 
   template <typename Adapter>
   Status GenerateOutput() {
@@ -768,7 +769,7 @@ struct SparseUnionSelectionImpl
           return Status::OK();
         },
         [&]() {
-          child_id_buffer_builder_.UnsafeAppend(type_codes_[0]);
+          child_id_buffer_builder_.UnsafeAppend(type_code_for_null_);
           return Status::OK();
         }));
     return Status::OK();

cpp/src/arrow/compute/kernels/vector_selection_test.cc

@@ -282,11 +282,6 @@ class TestFilterKernel : public ::testing::Test {
                     const std::shared_ptr<Array>& expected) {
     DoAssertFilter(values, filter, expected);
 
-    if (values->type_id() == Type::DENSE_UNION) {
-      // Concatenation of dense union not supported
-      return;
-    }
-
     // Check slicing: add M(=3) dummy values at the start and end of `values`,
     // add N(=2) dummy values at the start and end of `filter`.
     ARROW_SCOPED_TRACE("for sliced values and filter");
@@ -785,17 +780,6 @@ TEST_F(TestFilterKernelWithUnion, FilterUnion) {
       [2, null]
     ])");
     this->AssertFilter(union_type, union_json, "[1, 1, 1, 1, 1, 1, 1]", union_json);
-
-    // Sliced
-    // (check this manually as concatenation of dense unions isn't supported: ARROW-4975)
-    auto values = ArrayFromJSON(union_type, union_json)->Slice(2, 4);
-    auto filter = ArrayFromJSON(boolean(), "[0, 1, 1, null, 0, 1, 1]")->Slice(2, 4);
-    auto expected = ArrayFromJSON(union_type, R"([
-      [5, "hello"],
-      [2, null],
-      [2, 111]
-    ])");
-    this->AssertFilter(values, filter, expected);
   }
 }
 
@@ -1029,13 +1013,11 @@ void CheckTake(const std::shared_ptr<DataType>& type, const std::string& values_
     AssertTakeArrays(values, indices, expected);
 
     // Check sliced values
-    if (type->id() != Type::DENSE_UNION) {
-      ASSERT_OK_AND_ASSIGN(auto values_filler, MakeArrayOfNull(type, 2));
-      ASSERT_OK_AND_ASSIGN(auto values_sliced,
-                           Concatenate({values_filler, values, values_filler}));
-      values_sliced = values_sliced->Slice(2, values->length());
-      AssertTakeArrays(values_sliced, indices, expected);
-    }
+    ASSERT_OK_AND_ASSIGN(auto values_filler, MakeArrayOfNull(type, 2));
+    ASSERT_OK_AND_ASSIGN(auto values_sliced,
+                         Concatenate({values_filler, values, values_filler}));
+    values_sliced = values_sliced->Slice(2, values->length());
+    AssertTakeArrays(values_sliced, indices, expected);
 
     // Check sliced indices
     ASSERT_OK_AND_ASSIGN(auto zero, MakeScalar(index_type, int8_t{0}));
@@ -1484,30 +1466,30 @@ TEST_F(TestTakeKernelWithUnion, TakeUnion) {
        {dense_union({field("a", int32()), field("b", utf8())}, {2, 5}),
         sparse_union({field("a", int32()), field("b", utf8())}, {2, 5})}) {
     auto union_json = R"([
-      [2, null],
       [2, 222],
+      [2, null],
       [5, "hello"],
       [5, "eh"],
       [2, null],
       [2, 111],
       [5, null]
     ])";
     CheckTake(union_type, union_json, "[]", "[]");
-    CheckTake(union_type, union_json, "[3, 1, 3, 1, 3]", R"([
+    CheckTake(union_type, union_json, "[3, 0, 3, 0, 3]", R"([
       [5, "eh"],
       [2, 222],
       [5, "eh"],
       [2, 222],
       [5, "eh"]
     ])");
-    CheckTake(union_type, union_json, "[4, 2, 1, 6]", R"([
+    CheckTake(union_type, union_json, "[4, 2, 0, 6]", R"([
       [2, null],
       [5, "hello"],
       [2, 222],
       [5, null]
     ])");
     CheckTake(union_type, union_json, "[0, 1, 2, 3, 4, 5, 6]", union_json);
-    CheckTake(union_type, union_json, "[0, 2, 2, 2, 2, 2, 2]", R"([
+    CheckTake(union_type, union_json, "[1, 2, 2, 2, 2, 2, 2]", R"([
       [2, null],
       [5, "hello"],
       [5, "hello"],
@@ -1516,6 +1498,15 @@ TEST_F(TestTakeKernelWithUnion, TakeUnion) {
       [5, "hello"],
       [5, "hello"]
     ])");
+    CheckTake(union_type, union_json, "[0, null, 1, null, 2, 2, 2]", R"([
+      [2, 222],
+      [2, null],
+      [2, null],
+      [2, null],
+      [5, "hello"],
+      [5, "hello"],
+      [5, "hello"]
+    ])");
   }
 }