[GitHub] [arrow] bkietz commented on a change in pull request #7534: ARROW-8729: [C++][Dataset] Ensure non-empty batches when only virtual columns are projected

[GitBox <gi...@apache.org>]

bkietz commented on a change in pull request #7534:
URL: https://github.com/apache/arrow/pull/7534#discussion_r445470685



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File path: cpp/src/parquet/arrow/reader.cc
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@@ -338,22 +348,37 @@ class RowGroupRecordBatchReader : public ::arrow::RecordBatchReader {
     // TODO (hatemhelal): Consider refactoring this to share logic with ReadTable as this
     // does not currently honor the use_threads option.
     std::vector<std::shared_ptr<ChunkedArray>> columns(field_readers_.size());
-    for (size_t i = 0; i < field_readers_.size(); ++i) {
-      RETURN_NOT_OK(field_readers_[i]->NextBatch(batch_size_, &columns[i]));
-      if (columns[i]->num_chunks() > 1) {
-        return Status::NotImplemented("This class cannot yet iterate chunked arrays");
+    int64_t num_rows = -1;
+
+    if (columns.empty()) {
+      num_rows = std::min(batch_size_, *row_group_remaining_size_);

Review comment:
       ```suggestion
         // num_rows cannot be derived from field_readers_ so compute it using
         // row group sizes cached from metadata
         num_rows = std::min(batch_size_, *row_group_remaining_size_);
   ```




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