lidavidm commented on a change in pull request #10927:
URL: https://github.com/apache/arrow/pull/10927#discussion_r689712083



##########
File path: cpp/src/arrow/compute/exec/union_node.cc
##########
@@ -0,0 +1,150 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+#include <mutex>
+
+#include "arrow/api.h"
+#include "arrow/compute/api.h"
+#include "arrow/compute/exec/exec_plan.h"
+#include "arrow/compute/exec/options.h"
+#include "arrow/compute/exec/util.h"
+#include "arrow/util/bitmap_ops.h"
+#include "arrow/util/checked_cast.h"
+#include "arrow/util/future.h"
+#include "arrow/util/logging.h"
+#include "arrow/util/thread_pool.h"
+
+namespace arrow {
+
+using internal::checked_cast;
+
+namespace compute {
+
+namespace {
+std::vector<std::string> GetInputLabels(ExecNode::NodeVector inputs) {

Review comment:
       ```suggestion
   std::vector<std::string> GetInputLabels(const ExecNode::NodeVector& inputs) {
   ```

##########
File path: cpp/src/arrow/compute/exec/union_node.cc
##########
@@ -0,0 +1,150 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+#include <mutex>
+
+#include "arrow/api.h"
+#include "arrow/compute/api.h"
+#include "arrow/compute/exec/exec_plan.h"
+#include "arrow/compute/exec/options.h"
+#include "arrow/compute/exec/util.h"
+#include "arrow/util/bitmap_ops.h"
+#include "arrow/util/checked_cast.h"
+#include "arrow/util/future.h"
+#include "arrow/util/logging.h"
+#include "arrow/util/thread_pool.h"
+
+namespace arrow {
+
+using internal::checked_cast;
+
+namespace compute {
+
+namespace {
+std::vector<std::string> GetInputLabels(ExecNode::NodeVector inputs) {
+  std::vector<std::string> labels(inputs.size());
+  for (size_t i = 0; i < inputs.size(); i++) {
+    labels[i] = "input_" + std::to_string(i) + "_label";
+  }
+  return labels;
+}
+}  // namespace
+struct UnionNode : ExecNode {
+  UnionNode(ExecPlan* plan, std::vector<ExecNode*> inputs)
+      : ExecNode(plan, inputs, GetInputLabels(inputs),
+                 /*output_schema=*/inputs[0]->output_schema(),
+                 /*num_outputs=*/1) {
+    if (this->input_count_.SetTotal(static_cast<int>(inputs.size()))) {
+      finished_.MarkFinished();

Review comment:
       Hmm, this branch shouldn't be possible unless inputs == 0 which isn't 
allowed by Make.

##########
File path: cpp/src/arrow/compute/exec/union_node.cc
##########
@@ -0,0 +1,150 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+#include <mutex>
+
+#include "arrow/api.h"
+#include "arrow/compute/api.h"
+#include "arrow/compute/exec/exec_plan.h"
+#include "arrow/compute/exec/options.h"
+#include "arrow/compute/exec/util.h"
+#include "arrow/util/bitmap_ops.h"
+#include "arrow/util/checked_cast.h"
+#include "arrow/util/future.h"
+#include "arrow/util/logging.h"
+#include "arrow/util/thread_pool.h"
+
+namespace arrow {
+
+using internal::checked_cast;
+
+namespace compute {
+
+namespace {
+std::vector<std::string> GetInputLabels(ExecNode::NodeVector inputs) {
+  std::vector<std::string> labels(inputs.size());
+  for (size_t i = 0; i < inputs.size(); i++) {
+    labels[i] = "input_" + std::to_string(i) + "_label";
+  }
+  return labels;
+}
+}  // namespace
+struct UnionNode : ExecNode {
+  UnionNode(ExecPlan* plan, std::vector<ExecNode*> inputs)
+      : ExecNode(plan, inputs, GetInputLabels(inputs),
+                 /*output_schema=*/inputs[0]->output_schema(),
+                 /*num_outputs=*/1) {
+    if (this->input_count_.SetTotal(static_cast<int>(inputs.size()))) {
+      finished_.MarkFinished();
+    }
+  }
+
+  const char* kind_name() override { return "UnionNode"; }
+
+  static Result<ExecNode*> Make(ExecPlan* plan, std::vector<ExecNode*> inputs,
+                                const ExecNodeOptions& options) {
+    RETURN_NOT_OK(ValidateExecNodeInputs(plan, inputs, inputs.size(), 
"UnionNode"));
+    if (inputs.size() < 2) {
+      return Status::Invalid("Constructing a `UnionNode` with inputs size less 
than 2");
+    }
+    auto schema = inputs.at(0)->output_schema();
+    for (auto input : inputs) {
+      if (!input->output_schema()->Equals(schema)) {
+        return Status::Invalid(
+            "Constructing a `UnionNode` with inputs with different schemas");
+      }
+    }
+    return plan->EmplaceNode<UnionNode>(plan, std::move(inputs));
+  }
+
+  inline bool IsLeftInput(ExecNode* input) { return input == inputs_[0]; }
+
+  void InputReceived(ExecNode* input, int seq, ExecBatch batch) override {
+    ARROW_DCHECK(std::find_if(inputs_.begin(), inputs_.end(), 
[input](ExecNode* n) {
+                   return n == input;
+                 }) != inputs_.end());
+
+    if (finished_.is_finished()) {
+      return;
+    }
+    outputs_[0]->InputReceived(this, seq, std::move(batch));

Review comment:
       I wonder if it actually makes sense to forward seq like this. As I 
understand nodes can't really rely on this anyways so I don't think it harms 
things, but it seems odd to even have the parameter in that case. CC @bkietz 
for opinions. 
   
   (We could modify `Increment` to also give the count, if we wanted to give a 
unique seq_num to each batch we emit here.)

##########
File path: cpp/src/arrow/compute/exec/union_node.cc
##########
@@ -0,0 +1,150 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+#include <mutex>
+
+#include "arrow/api.h"
+#include "arrow/compute/api.h"
+#include "arrow/compute/exec/exec_plan.h"
+#include "arrow/compute/exec/options.h"
+#include "arrow/compute/exec/util.h"
+#include "arrow/util/bitmap_ops.h"
+#include "arrow/util/checked_cast.h"
+#include "arrow/util/future.h"
+#include "arrow/util/logging.h"
+#include "arrow/util/thread_pool.h"
+
+namespace arrow {
+
+using internal::checked_cast;
+
+namespace compute {
+
+namespace {
+std::vector<std::string> GetInputLabels(ExecNode::NodeVector inputs) {
+  std::vector<std::string> labels(inputs.size());
+  for (size_t i = 0; i < inputs.size(); i++) {
+    labels[i] = "input_" + std::to_string(i) + "_label";
+  }
+  return labels;
+}
+}  // namespace
+struct UnionNode : ExecNode {
+  UnionNode(ExecPlan* plan, std::vector<ExecNode*> inputs)
+      : ExecNode(plan, inputs, GetInputLabels(inputs),
+                 /*output_schema=*/inputs[0]->output_schema(),
+                 /*num_outputs=*/1) {
+    if (this->input_count_.SetTotal(static_cast<int>(inputs.size()))) {
+      finished_.MarkFinished();
+    }
+  }
+
+  const char* kind_name() override { return "UnionNode"; }
+
+  static Result<ExecNode*> Make(ExecPlan* plan, std::vector<ExecNode*> inputs,
+                                const ExecNodeOptions& options) {
+    RETURN_NOT_OK(ValidateExecNodeInputs(plan, inputs, inputs.size(), 
"UnionNode"));
+    if (inputs.size() < 2) {
+      return Status::Invalid("Constructing a `UnionNode` with inputs size less 
than 2");
+    }
+    auto schema = inputs.at(0)->output_schema();
+    for (auto input : inputs) {
+      if (!input->output_schema()->Equals(schema)) {
+        return Status::Invalid(
+            "Constructing a `UnionNode` with inputs with different schemas");

Review comment:
       Nit: it might be nice to provide the schemas to aid debugging, something 
like
   
   ```cpp
   return Status::Invalid("UnionNode input schemas must all match, first schema 
was: ", *schema, " got schema: ", *input->output_schema);
   ```

##########
File path: cpp/src/arrow/compute/exec/union_node.cc
##########
@@ -0,0 +1,150 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+#include <mutex>
+
+#include "arrow/api.h"
+#include "arrow/compute/api.h"
+#include "arrow/compute/exec/exec_plan.h"
+#include "arrow/compute/exec/options.h"
+#include "arrow/compute/exec/util.h"
+#include "arrow/util/bitmap_ops.h"
+#include "arrow/util/checked_cast.h"
+#include "arrow/util/future.h"
+#include "arrow/util/logging.h"
+#include "arrow/util/thread_pool.h"
+
+namespace arrow {
+
+using internal::checked_cast;
+
+namespace compute {
+
+namespace {
+std::vector<std::string> GetInputLabels(ExecNode::NodeVector inputs) {
+  std::vector<std::string> labels(inputs.size());
+  for (size_t i = 0; i < inputs.size(); i++) {
+    labels[i] = "input_" + std::to_string(i) + "_label";
+  }
+  return labels;
+}
+}  // namespace
+struct UnionNode : ExecNode {
+  UnionNode(ExecPlan* plan, std::vector<ExecNode*> inputs)
+      : ExecNode(plan, inputs, GetInputLabels(inputs),
+                 /*output_schema=*/inputs[0]->output_schema(),
+                 /*num_outputs=*/1) {
+    if (this->input_count_.SetTotal(static_cast<int>(inputs.size()))) {
+      finished_.MarkFinished();
+    }
+  }
+
+  const char* kind_name() override { return "UnionNode"; }
+
+  static Result<ExecNode*> Make(ExecPlan* plan, std::vector<ExecNode*> inputs,
+                                const ExecNodeOptions& options) {
+    RETURN_NOT_OK(ValidateExecNodeInputs(plan, inputs, inputs.size(), 
"UnionNode"));
+    if (inputs.size() < 2) {
+      return Status::Invalid("Constructing a `UnionNode` with inputs size less 
than 2");
+    }
+    auto schema = inputs.at(0)->output_schema();
+    for (auto input : inputs) {
+      if (!input->output_schema()->Equals(schema)) {
+        return Status::Invalid(
+            "Constructing a `UnionNode` with inputs with different schemas");
+      }
+    }
+    return plan->EmplaceNode<UnionNode>(plan, std::move(inputs));
+  }
+
+  inline bool IsLeftInput(ExecNode* input) { return input == inputs_[0]; }
+
+  void InputReceived(ExecNode* input, int seq, ExecBatch batch) override {
+    ARROW_DCHECK(std::find_if(inputs_.begin(), inputs_.end(), 
[input](ExecNode* n) {
+                   return n == input;
+                 }) != inputs_.end());
+
+    if (finished_.is_finished()) {
+      return;
+    }
+    outputs_[0]->InputReceived(this, seq, std::move(batch));
+    if (batch_count_.Increment()) {
+      finished_.MarkFinished();
+    }
+  }
+
+  void ErrorReceived(ExecNode* input, Status error) override {
+    DCHECK_EQ(input, inputs_[0]);
+    outputs_[0]->ErrorReceived(this, std::move(error));
+
+    StopProducing();
+  }
+
+  void InputFinished(ExecNode* input, int num_total) override {
+    ARROW_DCHECK(std::find_if(inputs_.begin(), inputs_.end(), 
[input](ExecNode* n) {
+                   return n == input;
+                 }) != inputs_.end());
+
+    total_batches_.fetch_add(num_total);
+
+    if (input_count_.Increment()) {
+      outputs_[0]->InputFinished(this, total_batches_.load());
+      if (batch_count_.SetTotal(total_batches_.load())) {
+        finished_.MarkFinished();
+      }
+    }
+  }
+
+  Status StartProducing() override {
+    finished_ = Future<>::Make();
+    return Status::OK();
+  }
+
+  void PauseProducing(ExecNode* output) override {}
+
+  void ResumeProducing(ExecNode* output) override {}

Review comment:
       Or if not, we should make a followup JIRA.

##########
File path: cpp/src/arrow/compute/exec/union_node.cc
##########
@@ -0,0 +1,150 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+#include <mutex>
+
+#include "arrow/api.h"
+#include "arrow/compute/api.h"
+#include "arrow/compute/exec/exec_plan.h"
+#include "arrow/compute/exec/options.h"
+#include "arrow/compute/exec/util.h"
+#include "arrow/util/bitmap_ops.h"
+#include "arrow/util/checked_cast.h"
+#include "arrow/util/future.h"
+#include "arrow/util/logging.h"
+#include "arrow/util/thread_pool.h"
+
+namespace arrow {
+
+using internal::checked_cast;
+
+namespace compute {
+
+namespace {
+std::vector<std::string> GetInputLabels(ExecNode::NodeVector inputs) {
+  std::vector<std::string> labels(inputs.size());
+  for (size_t i = 0; i < inputs.size(); i++) {
+    labels[i] = "input_" + std::to_string(i) + "_label";
+  }
+  return labels;
+}
+}  // namespace
+struct UnionNode : ExecNode {
+  UnionNode(ExecPlan* plan, std::vector<ExecNode*> inputs)
+      : ExecNode(plan, inputs, GetInputLabels(inputs),
+                 /*output_schema=*/inputs[0]->output_schema(),
+                 /*num_outputs=*/1) {
+    if (this->input_count_.SetTotal(static_cast<int>(inputs.size()))) {
+      finished_.MarkFinished();
+    }
+  }
+
+  const char* kind_name() override { return "UnionNode"; }
+
+  static Result<ExecNode*> Make(ExecPlan* plan, std::vector<ExecNode*> inputs,
+                                const ExecNodeOptions& options) {
+    RETURN_NOT_OK(ValidateExecNodeInputs(plan, inputs, inputs.size(), 
"UnionNode"));
+    if (inputs.size() < 2) {
+      return Status::Invalid("Constructing a `UnionNode` with inputs size less 
than 2");
+    }
+    auto schema = inputs.at(0)->output_schema();
+    for (auto input : inputs) {
+      if (!input->output_schema()->Equals(schema)) {
+        return Status::Invalid(
+            "Constructing a `UnionNode` with inputs with different schemas");
+      }
+    }
+    return plan->EmplaceNode<UnionNode>(plan, std::move(inputs));
+  }
+
+  inline bool IsLeftInput(ExecNode* input) { return input == inputs_[0]; }

Review comment:
       This looks unused now.

##########
File path: cpp/src/arrow/compute/exec/union_node_test.cc
##########
@@ -0,0 +1,203 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+#include <gmock/gmock-matchers.h>
+
+#include <iostream>
+
+#include "arrow/api.h"
+#include "arrow/compute/exec/options.h"
+#include "arrow/compute/exec/test_util.h"
+#include "arrow/pretty_print.h"
+#include "arrow/testing/gtest_util.h"
+#include "arrow/testing/matchers.h"
+
+using testing::UnorderedElementsAreArray;
+
+namespace arrow {
+namespace compute {
+
+void GenerateBatchesFromString(const std::shared_ptr<Schema>& schema,
+                               const std::vector<util::string_view>& 
json_strings,
+                               BatchesWithSchema* out_batches, int 
multiplicity = 1) {
+  std::vector<ValueDescr> descrs;
+  for (auto&& field : schema->fields()) {
+    descrs.emplace_back(field->type());
+  }
+
+  for (auto&& s : json_strings) {
+    out_batches->batches.push_back(ExecBatchFromJSON(descrs, s));
+  }
+
+  size_t batch_count = out_batches->batches.size();
+  for (int repeat = 1; repeat < multiplicity; ++repeat) {
+    for (size_t i = 0; i < batch_count; ++i) {
+      out_batches->batches.push_back(out_batches->batches[i]);
+    }
+  }
+
+  out_batches->schema = schema;
+}
+
+void CheckRunOutput(const BatchesWithSchema& l_batches,
+                    const BatchesWithSchema& r_batches,
+                    const BatchesWithSchema& exp_batches, bool parallel = 
false) {
+  SCOPED_TRACE(parallel ? "parallel" : "single threaded");
+
+  ASSERT_OK_AND_ASSIGN(auto plan, ExecPlan::Make());
+
+  Declaration union_decl{"union", ExecNodeOptions{}};
+
+  // add left source
+  union_decl.inputs.emplace_back(Declaration{
+      "source", SourceNodeOptions{l_batches.schema, l_batches.gen(parallel,
+                                                                  
/*slow=*/false)}});
+  // add right source
+  union_decl.inputs.emplace_back(Declaration{
+      "source", SourceNodeOptions{r_batches.schema, r_batches.gen(parallel,
+                                                                  
/*slow=*/false)}});
+  AsyncGenerator<util::optional<ExecBatch>> sink_gen;
+
+  ASSERT_OK(Declaration::Sequence({union_decl, {"sink", 
SinkNodeOptions{&sink_gen}}})
+                .AddToPlan(plan.get()));
+
+  Future<std::vector<ExecBatch>> actual = StartAndCollect(plan.get(), 
sink_gen);
+
+  auto expected_matcher =
+      Finishes(ResultWith(UnorderedElementsAreArray(exp_batches.batches)));
+  ASSERT_THAT(actual, expected_matcher);
+}
+
+void RunNonEmptyTest(bool parallel) {
+  auto l_schema = schema({field("colum_i32", int32()), field("colum_str", 
utf8())});
+  auto r_schema = schema({field("colum_i32", int32()), field("colum_str", 
utf8())});
+  BatchesWithSchema l_batches, r_batches, exp_batches;
+
+  int multiplicity = parallel ? 100 : 1;
+
+  GenerateBatchesFromString(l_schema,
+                            {
+                                R"([[0,"d"], [1,"b"]])",
+                                R"([[2,"d"], [3,"a"], [4,"a"]])",
+                            },
+                            &l_batches, multiplicity);
+
+  GenerateBatchesFromString(r_schema,
+                            {
+                                R"([[10,"A"]])",
+                            },
+                            &r_batches, multiplicity);
+
+  GenerateBatchesFromString(l_schema,
+                            {
+                                R"([[0,"d"], [1,"b"]])",
+                                R"([[2,"d"], [3,"a"], [4,"a"]])",
+
+                                R"([[10,"A"]])",
+                            },
+                            &exp_batches, multiplicity);
+  CheckRunOutput(l_batches, r_batches, exp_batches, parallel);
+}
+
+void RunEmptyTest(bool parallel) {
+  auto l_schema = schema({field("colum_i32", int32()), field("colum_str", 
utf8())});
+  auto r_schema = schema({field("colum_i32", int32()), field("colum_str", 
utf8())});
+
+  int multiplicity = parallel ? 100 : 1;
+
+  BatchesWithSchema l_empty, r_empty, output_batches;
+
+  GenerateBatchesFromString(l_schema, {R"([])"}, &l_empty, multiplicity);
+  GenerateBatchesFromString(r_schema, {R"([])"}, &r_empty, multiplicity);
+
+  GenerateBatchesFromString(l_schema, {R"([])", R"([])"}, &output_batches, 
multiplicity);
+
+  CheckRunOutput(l_empty, r_empty, output_batches);
+}
+
+TEST(UnionTest, TestNonEmpty) {
+  for (bool parallel : {false, true}) {
+    RunNonEmptyTest(parallel);
+  }
+}
+
+TEST(UnionTest, TestEmpty) {
+  for (bool parallel : {false, true}) {
+    RunEmptyTest(parallel);
+  }
+}
+
+void TestUnionRandom(const std::shared_ptr<DataType>& data_type, bool parallel,
+                     int num_batches, int batch_size) {
+  auto l_schema = schema({field("colum0", data_type), field("colum1", 
data_type)});
+  auto r_schema = schema({field("colum0", data_type), field("colum1", 
data_type)});
+
+  // generate data
+  auto l_batches = MakeRandomBatches(l_schema, num_batches, batch_size);
+  auto r_batches = MakeRandomBatches(r_schema, num_batches, batch_size);
+
+  ASSERT_OK_AND_ASSIGN(auto plan, ExecPlan::Make());
+
+  Declaration Union{"union", ExecNodeOptions{}};
+
+  // add left source
+  Union.inputs.emplace_back(Declaration{
+      "source", SourceNodeOptions{l_batches.schema, l_batches.gen(parallel,
+                                                                  
/*slow=*/false)}});
+  // add right source
+  Union.inputs.emplace_back(Declaration{
+      "source", SourceNodeOptions{r_batches.schema, r_batches.gen(parallel,

Review comment:
       Maybe we could make this test use three inputs? We could even just 
duplicate one of the existing inputs.

##########
File path: cpp/src/arrow/compute/exec/union_node.cc
##########
@@ -0,0 +1,150 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+#include <mutex>
+
+#include "arrow/api.h"
+#include "arrow/compute/api.h"
+#include "arrow/compute/exec/exec_plan.h"
+#include "arrow/compute/exec/options.h"
+#include "arrow/compute/exec/util.h"
+#include "arrow/util/bitmap_ops.h"
+#include "arrow/util/checked_cast.h"
+#include "arrow/util/future.h"
+#include "arrow/util/logging.h"
+#include "arrow/util/thread_pool.h"
+
+namespace arrow {
+
+using internal::checked_cast;
+
+namespace compute {
+
+namespace {
+std::vector<std::string> GetInputLabels(ExecNode::NodeVector inputs) {
+  std::vector<std::string> labels(inputs.size());
+  for (size_t i = 0; i < inputs.size(); i++) {
+    labels[i] = "input_" + std::to_string(i) + "_label";
+  }
+  return labels;
+}
+}  // namespace
+struct UnionNode : ExecNode {
+  UnionNode(ExecPlan* plan, std::vector<ExecNode*> inputs)
+      : ExecNode(plan, inputs, GetInputLabels(inputs),
+                 /*output_schema=*/inputs[0]->output_schema(),
+                 /*num_outputs=*/1) {
+    if (this->input_count_.SetTotal(static_cast<int>(inputs.size()))) {
+      finished_.MarkFinished();
+    }
+  }
+
+  const char* kind_name() override { return "UnionNode"; }
+
+  static Result<ExecNode*> Make(ExecPlan* plan, std::vector<ExecNode*> inputs,
+                                const ExecNodeOptions& options) {
+    RETURN_NOT_OK(ValidateExecNodeInputs(plan, inputs, inputs.size(), 
"UnionNode"));
+    if (inputs.size() < 2) {
+      return Status::Invalid("Constructing a `UnionNode` with inputs size less 
than 2");
+    }
+    auto schema = inputs.at(0)->output_schema();
+    for (auto input : inputs) {
+      if (!input->output_schema()->Equals(schema)) {
+        return Status::Invalid(
+            "Constructing a `UnionNode` with inputs with different schemas");
+      }
+    }
+    return plan->EmplaceNode<UnionNode>(plan, std::move(inputs));
+  }
+
+  inline bool IsLeftInput(ExecNode* input) { return input == inputs_[0]; }
+
+  void InputReceived(ExecNode* input, int seq, ExecBatch batch) override {
+    ARROW_DCHECK(std::find_if(inputs_.begin(), inputs_.end(), 
[input](ExecNode* n) {
+                   return n == input;
+                 }) != inputs_.end());
+
+    if (finished_.is_finished()) {
+      return;
+    }
+    outputs_[0]->InputReceived(this, seq, std::move(batch));
+    if (batch_count_.Increment()) {
+      finished_.MarkFinished();
+    }
+  }
+
+  void ErrorReceived(ExecNode* input, Status error) override {
+    DCHECK_EQ(input, inputs_[0]);
+    outputs_[0]->ErrorReceived(this, std::move(error));
+
+    StopProducing();
+  }
+
+  void InputFinished(ExecNode* input, int num_total) override {
+    ARROW_DCHECK(std::find_if(inputs_.begin(), inputs_.end(), 
[input](ExecNode* n) {
+                   return n == input;
+                 }) != inputs_.end());
+
+    total_batches_.fetch_add(num_total);
+
+    if (input_count_.Increment()) {
+      outputs_[0]->InputFinished(this, total_batches_.load());
+      if (batch_count_.SetTotal(total_batches_.load())) {
+        finished_.MarkFinished();
+      }
+    }
+  }
+
+  Status StartProducing() override {
+    finished_ = Future<>::Make();
+    return Status::OK();
+  }
+
+  void PauseProducing(ExecNode* output) override {}
+
+  void ResumeProducing(ExecNode* output) override {}
+
+  void StopProducing(ExecNode* output) override {
+    DCHECK_EQ(output, outputs_[0]);
+    if (batch_count_.Cancel()) {
+      finished_.MakeFinished();

Review comment:
       ```suggestion
         finished_.MarkFinished();
   ```

##########
File path: cpp/src/arrow/compute/exec/union_node.cc
##########
@@ -0,0 +1,150 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+#include <mutex>
+
+#include "arrow/api.h"
+#include "arrow/compute/api.h"
+#include "arrow/compute/exec/exec_plan.h"
+#include "arrow/compute/exec/options.h"
+#include "arrow/compute/exec/util.h"
+#include "arrow/util/bitmap_ops.h"
+#include "arrow/util/checked_cast.h"
+#include "arrow/util/future.h"
+#include "arrow/util/logging.h"
+#include "arrow/util/thread_pool.h"
+
+namespace arrow {
+
+using internal::checked_cast;
+
+namespace compute {
+
+namespace {
+std::vector<std::string> GetInputLabels(ExecNode::NodeVector inputs) {
+  std::vector<std::string> labels(inputs.size());
+  for (size_t i = 0; i < inputs.size(); i++) {
+    labels[i] = "input_" + std::to_string(i) + "_label";
+  }
+  return labels;
+}
+}  // namespace
+struct UnionNode : ExecNode {
+  UnionNode(ExecPlan* plan, std::vector<ExecNode*> inputs)
+      : ExecNode(plan, inputs, GetInputLabels(inputs),
+                 /*output_schema=*/inputs[0]->output_schema(),
+                 /*num_outputs=*/1) {
+    if (this->input_count_.SetTotal(static_cast<int>(inputs.size()))) {
+      finished_.MarkFinished();
+    }
+  }
+
+  const char* kind_name() override { return "UnionNode"; }
+
+  static Result<ExecNode*> Make(ExecPlan* plan, std::vector<ExecNode*> inputs,
+                                const ExecNodeOptions& options) {
+    RETURN_NOT_OK(ValidateExecNodeInputs(plan, inputs, inputs.size(), 
"UnionNode"));
+    if (inputs.size() < 2) {
+      return Status::Invalid("Constructing a `UnionNode` with inputs size less 
than 2");
+    }
+    auto schema = inputs.at(0)->output_schema();
+    for (auto input : inputs) {
+      if (!input->output_schema()->Equals(schema)) {
+        return Status::Invalid(
+            "Constructing a `UnionNode` with inputs with different schemas");
+      }
+    }
+    return plan->EmplaceNode<UnionNode>(plan, std::move(inputs));
+  }
+
+  inline bool IsLeftInput(ExecNode* input) { return input == inputs_[0]; }
+
+  void InputReceived(ExecNode* input, int seq, ExecBatch batch) override {
+    ARROW_DCHECK(std::find_if(inputs_.begin(), inputs_.end(), 
[input](ExecNode* n) {
+                   return n == input;
+                 }) != inputs_.end());
+
+    if (finished_.is_finished()) {
+      return;
+    }
+    outputs_[0]->InputReceived(this, seq, std::move(batch));
+    if (batch_count_.Increment()) {
+      finished_.MarkFinished();
+    }
+  }
+
+  void ErrorReceived(ExecNode* input, Status error) override {
+    DCHECK_EQ(input, inputs_[0]);
+    outputs_[0]->ErrorReceived(this, std::move(error));
+
+    StopProducing();
+  }
+
+  void InputFinished(ExecNode* input, int num_total) override {
+    ARROW_DCHECK(std::find_if(inputs_.begin(), inputs_.end(), 
[input](ExecNode* n) {
+                   return n == input;
+                 }) != inputs_.end());
+
+    total_batches_.fetch_add(num_total);
+
+    if (input_count_.Increment()) {
+      outputs_[0]->InputFinished(this, total_batches_.load());
+      if (batch_count_.SetTotal(total_batches_.load())) {
+        finished_.MarkFinished();
+      }
+    }
+  }
+
+  Status StartProducing() override {
+    finished_ = Future<>::Make();
+    return Status::OK();
+  }
+
+  void PauseProducing(ExecNode* output) override {}
+
+  void ResumeProducing(ExecNode* output) override {}

Review comment:
       I think we can trivially forward pause/resume to our inputs.

##########
File path: cpp/src/arrow/compute/exec/union_node.cc
##########
@@ -0,0 +1,150 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+#include <mutex>
+
+#include "arrow/api.h"
+#include "arrow/compute/api.h"
+#include "arrow/compute/exec/exec_plan.h"
+#include "arrow/compute/exec/options.h"
+#include "arrow/compute/exec/util.h"
+#include "arrow/util/bitmap_ops.h"
+#include "arrow/util/checked_cast.h"
+#include "arrow/util/future.h"
+#include "arrow/util/logging.h"
+#include "arrow/util/thread_pool.h"
+
+namespace arrow {
+
+using internal::checked_cast;
+
+namespace compute {
+
+namespace {
+std::vector<std::string> GetInputLabels(ExecNode::NodeVector inputs) {
+  std::vector<std::string> labels(inputs.size());
+  for (size_t i = 0; i < inputs.size(); i++) {
+    labels[i] = "input_" + std::to_string(i) + "_label";
+  }
+  return labels;
+}
+}  // namespace
+struct UnionNode : ExecNode {
+  UnionNode(ExecPlan* plan, std::vector<ExecNode*> inputs)
+      : ExecNode(plan, inputs, GetInputLabels(inputs),
+                 /*output_schema=*/inputs[0]->output_schema(),
+                 /*num_outputs=*/1) {
+    if (this->input_count_.SetTotal(static_cast<int>(inputs.size()))) {
+      finished_.MarkFinished();
+    }
+  }
+
+  const char* kind_name() override { return "UnionNode"; }
+
+  static Result<ExecNode*> Make(ExecPlan* plan, std::vector<ExecNode*> inputs,
+                                const ExecNodeOptions& options) {
+    RETURN_NOT_OK(ValidateExecNodeInputs(plan, inputs, inputs.size(), 
"UnionNode"));
+    if (inputs.size() < 2) {
+      return Status::Invalid("Constructing a `UnionNode` with inputs size less 
than 2");
+    }
+    auto schema = inputs.at(0)->output_schema();
+    for (auto input : inputs) {
+      if (!input->output_schema()->Equals(schema)) {
+        return Status::Invalid(
+            "Constructing a `UnionNode` with inputs with different schemas");
+      }
+    }
+    return plan->EmplaceNode<UnionNode>(plan, std::move(inputs));
+  }
+
+  inline bool IsLeftInput(ExecNode* input) { return input == inputs_[0]; }
+
+  void InputReceived(ExecNode* input, int seq, ExecBatch batch) override {
+    ARROW_DCHECK(std::find_if(inputs_.begin(), inputs_.end(), 
[input](ExecNode* n) {
+                   return n == input;
+                 }) != inputs_.end());
+
+    if (finished_.is_finished()) {
+      return;
+    }
+    outputs_[0]->InputReceived(this, seq, std::move(batch));
+    if (batch_count_.Increment()) {
+      finished_.MarkFinished();
+    }
+  }
+
+  void ErrorReceived(ExecNode* input, Status error) override {
+    DCHECK_EQ(input, inputs_[0]);
+    outputs_[0]->ErrorReceived(this, std::move(error));
+
+    StopProducing();
+  }
+
+  void InputFinished(ExecNode* input, int num_total) override {
+    ARROW_DCHECK(std::find_if(inputs_.begin(), inputs_.end(), 
[input](ExecNode* n) {
+                   return n == input;
+                 }) != inputs_.end());
+
+    total_batches_.fetch_add(num_total);
+
+    if (input_count_.Increment()) {
+      outputs_[0]->InputFinished(this, total_batches_.load());
+      if (batch_count_.SetTotal(total_batches_.load())) {
+        finished_.MarkFinished();
+      }
+    }
+  }
+
+  Status StartProducing() override {
+    finished_ = Future<>::Make();
+    return Status::OK();
+  }
+
+  void PauseProducing(ExecNode* output) override {}
+
+  void ResumeProducing(ExecNode* output) override {}
+
+  void StopProducing(ExecNode* output) override {
+    DCHECK_EQ(output, outputs_[0]);
+    if (batch_count_.Cancel()) {
+      finished_.MakeFinished();
+    }
+    for (auto&& input : inputs_) {
+      input->StopProducing(this);
+    }
+  }
+
+  void StopProducing() override {
+    if (batch_count_.Cancel()) {
+      finished_.MakeFinished();
+    }
+    inputs_[0]->StopProducing(this);
+  }

Review comment:
       This should call StopProducing on every input.

##########
File path: cpp/src/arrow/compute/exec/union_node.cc
##########
@@ -0,0 +1,150 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+#include <mutex>
+
+#include "arrow/api.h"
+#include "arrow/compute/api.h"
+#include "arrow/compute/exec/exec_plan.h"
+#include "arrow/compute/exec/options.h"
+#include "arrow/compute/exec/util.h"
+#include "arrow/util/bitmap_ops.h"
+#include "arrow/util/checked_cast.h"
+#include "arrow/util/future.h"
+#include "arrow/util/logging.h"
+#include "arrow/util/thread_pool.h"
+
+namespace arrow {
+
+using internal::checked_cast;
+
+namespace compute {
+
+namespace {
+std::vector<std::string> GetInputLabels(ExecNode::NodeVector inputs) {
+  std::vector<std::string> labels(inputs.size());
+  for (size_t i = 0; i < inputs.size(); i++) {
+    labels[i] = "input_" + std::to_string(i) + "_label";
+  }
+  return labels;
+}
+}  // namespace
+struct UnionNode : ExecNode {
+  UnionNode(ExecPlan* plan, std::vector<ExecNode*> inputs)
+      : ExecNode(plan, inputs, GetInputLabels(inputs),
+                 /*output_schema=*/inputs[0]->output_schema(),
+                 /*num_outputs=*/1) {
+    if (this->input_count_.SetTotal(static_cast<int>(inputs.size()))) {
+      finished_.MarkFinished();
+    }
+  }
+
+  const char* kind_name() override { return "UnionNode"; }
+
+  static Result<ExecNode*> Make(ExecPlan* plan, std::vector<ExecNode*> inputs,
+                                const ExecNodeOptions& options) {
+    RETURN_NOT_OK(ValidateExecNodeInputs(plan, inputs, inputs.size(), 
"UnionNode"));
+    if (inputs.size() < 2) {

Review comment:
       It's kinda pointless, but allowing one input would be a little more 
consistent. (It might make life a little easier to not have to special case the 
n = 1 case when generating exec plans.)

##########
File path: cpp/src/arrow/compute/exec/union_node_test.cc
##########
@@ -0,0 +1,203 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+#include <gmock/gmock-matchers.h>
+
+#include <iostream>
+
+#include "arrow/api.h"
+#include "arrow/compute/exec/options.h"
+#include "arrow/compute/exec/test_util.h"
+#include "arrow/pretty_print.h"
+#include "arrow/testing/gtest_util.h"
+#include "arrow/testing/matchers.h"
+
+using testing::UnorderedElementsAreArray;
+
+namespace arrow {
+namespace compute {
+
+void GenerateBatchesFromString(const std::shared_ptr<Schema>& schema,
+                               const std::vector<util::string_view>& 
json_strings,
+                               BatchesWithSchema* out_batches, int 
multiplicity = 1) {
+  std::vector<ValueDescr> descrs;
+  for (auto&& field : schema->fields()) {
+    descrs.emplace_back(field->type());
+  }
+
+  for (auto&& s : json_strings) {
+    out_batches->batches.push_back(ExecBatchFromJSON(descrs, s));
+  }
+
+  size_t batch_count = out_batches->batches.size();
+  for (int repeat = 1; repeat < multiplicity; ++repeat) {
+    for (size_t i = 0; i < batch_count; ++i) {
+      out_batches->batches.push_back(out_batches->batches[i]);
+    }
+  }
+
+  out_batches->schema = schema;
+}
+
+void CheckRunOutput(const BatchesWithSchema& l_batches,
+                    const BatchesWithSchema& r_batches,
+                    const BatchesWithSchema& exp_batches, bool parallel = 
false) {
+  SCOPED_TRACE(parallel ? "parallel" : "single threaded");
+
+  ASSERT_OK_AND_ASSIGN(auto plan, ExecPlan::Make());
+
+  Declaration union_decl{"union", ExecNodeOptions{}};
+
+  // add left source
+  union_decl.inputs.emplace_back(Declaration{
+      "source", SourceNodeOptions{l_batches.schema, l_batches.gen(parallel,
+                                                                  
/*slow=*/false)}});
+  // add right source
+  union_decl.inputs.emplace_back(Declaration{
+      "source", SourceNodeOptions{r_batches.schema, r_batches.gen(parallel,
+                                                                  
/*slow=*/false)}});
+  AsyncGenerator<util::optional<ExecBatch>> sink_gen;
+
+  ASSERT_OK(Declaration::Sequence({union_decl, {"sink", 
SinkNodeOptions{&sink_gen}}})
+                .AddToPlan(plan.get()));
+
+  Future<std::vector<ExecBatch>> actual = StartAndCollect(plan.get(), 
sink_gen);
+
+  auto expected_matcher =
+      Finishes(ResultWith(UnorderedElementsAreArray(exp_batches.batches)));
+  ASSERT_THAT(actual, expected_matcher);
+}
+
+void RunNonEmptyTest(bool parallel) {
+  auto l_schema = schema({field("colum_i32", int32()), field("colum_str", 
utf8())});
+  auto r_schema = schema({field("colum_i32", int32()), field("colum_str", 
utf8())});
+  BatchesWithSchema l_batches, r_batches, exp_batches;
+
+  int multiplicity = parallel ? 100 : 1;
+
+  GenerateBatchesFromString(l_schema,
+                            {
+                                R"([[0,"d"], [1,"b"]])",
+                                R"([[2,"d"], [3,"a"], [4,"a"]])",
+                            },
+                            &l_batches, multiplicity);
+
+  GenerateBatchesFromString(r_schema,
+                            {
+                                R"([[10,"A"]])",
+                            },
+                            &r_batches, multiplicity);
+
+  GenerateBatchesFromString(l_schema,
+                            {
+                                R"([[0,"d"], [1,"b"]])",
+                                R"([[2,"d"], [3,"a"], [4,"a"]])",
+
+                                R"([[10,"A"]])",
+                            },
+                            &exp_batches, multiplicity);
+  CheckRunOutput(l_batches, r_batches, exp_batches, parallel);
+}
+
+void RunEmptyTest(bool parallel) {
+  auto l_schema = schema({field("colum_i32", int32()), field("colum_str", 
utf8())});
+  auto r_schema = schema({field("colum_i32", int32()), field("colum_str", 
utf8())});
+
+  int multiplicity = parallel ? 100 : 1;
+
+  BatchesWithSchema l_empty, r_empty, output_batches;
+
+  GenerateBatchesFromString(l_schema, {R"([])"}, &l_empty, multiplicity);
+  GenerateBatchesFromString(r_schema, {R"([])"}, &r_empty, multiplicity);
+
+  GenerateBatchesFromString(l_schema, {R"([])", R"([])"}, &output_batches, 
multiplicity);
+
+  CheckRunOutput(l_empty, r_empty, output_batches);
+}
+
+TEST(UnionTest, TestNonEmpty) {
+  for (bool parallel : {false, true}) {
+    RunNonEmptyTest(parallel);
+  }
+}
+
+TEST(UnionTest, TestEmpty) {
+  for (bool parallel : {false, true}) {
+    RunEmptyTest(parallel);
+  }
+}

Review comment:
       I think it'd be good to add a test that tries to create a union node 
with 0 inputs, and with differing schemas, to ensure those cases are covered.




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