add reserve and assert recordbatch

cpp/src/arrow/record_batch.cc

@@ -452,8 +452,10 @@ Result<std::shared_ptr<RecordBatch>> ConcatenateRecordBatches(
   }
 
   std::vector<std::shared_ptr<Array>> concatenated_columns;
+  concatenated_columns.reserve(cols);
   for (int col = 0; col < cols; ++col) {
     ArrayVector column_arrays;
+    column_arrays.reserve(batches.size());
     for (const auto& batch : batches) {
       column_arrays.emplace_back(batch->column(col));
     }

cpp/src/arrow/record_batch.h

@@ -352,6 +352,11 @@ class ARROW_EXPORT RecordBatchReader {
 
 /// \brief Concatenate record batches
 ///
+/// The columns of the new batch are formed by concatenate the same columns of each input
+/// batch. Concatenate multiple batches into a new batch requires that the schema must be
+/// consistent. It supports merging batches without columns (only length, scenarios such
+/// as count(*)).
+///
 /// \param[in] batches a vector of record batches to be concatenated
 /// \param[in] pool memory to store the result will be allocated from this memory pool
 /// \return the concatenated record batch

cpp/src/arrow/record_batch_test.cc

@@ -573,6 +573,8 @@ TEST_F(TestRecordBatch, ConcatenateRecordBatches) {
 
   ASSERT_OK_AND_ASSIGN(auto batch, ConcatenateRecordBatches({b1, b2}));
   ASSERT_EQ(batch->num_rows(), b1->num_rows() + b2->num_rows());
+  ASSERT_BATCHES_EQUAL(*batch->Slice(0, b1->num_rows()), *b1);
+  ASSERT_BATCHES_EQUAL(*batch->Slice(b1->num_rows()), *b2);
 
   f0 = field("fd0", int32());
   f1 = field("fd1", uint8());
@@ -587,6 +589,7 @@ TEST_F(TestRecordBatch, ConcatenateRecordBatches) {
                                       std::vector<std::shared_ptr<ArrayData>>{});
   ASSERT_OK_AND_ASSIGN(batch, ConcatenateRecordBatches({null_batch}));
   ASSERT_EQ(batch->num_rows(), null_batch->num_rows());
+  ASSERT_BATCHES_EQUAL(*batch, *null_batch);
 }
 
 }  // namespace arrow