[GitHub] [arrow] benibus commented on a diff in pull request #35197: GH-14946: [C++] Add flattening FieldPath/FieldRef::Get methods

["benibus (via GitHub)" <gi...@apache.org>]

benibus commented on code in PR #35197:
URL: https://github.com/apache/arrow/pull/35197#discussion_r1175670935


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cpp/src/arrow/type.cc:
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@@ -1129,18 +1129,20 @@ class ChunkedArrayData : public ChunkedColumn {
 // Return a vector of ChunkedColumns - one for each struct field.
 // Unlike ChunkedArray::Flatten, this is zero-copy and doesn't merge parent/child
 // validity bitmaps.
-ChunkedColumnVector ChunkedColumn::Flatten() const {
+ChunkedColumnVector ChunkedColumn::FlattenZeroCopy() const {
   DCHECK_EQ(type()->id(), Type::STRUCT);
 
   ChunkedColumnVector columns(type()->num_fields());
   for (int column_idx = 0; column_idx < type()->num_fields(); ++column_idx) {
     const auto& child_type = type()->field(column_idx)->type();
     ArrayDataVector chunks(num_chunks());
     for (int chunk_idx = 0; chunk_idx < num_chunks(); ++chunk_idx) {
-      const auto& child_data = chunk(chunk_idx)->child_data;
-      DCHECK_EQ(columns.size(), child_data.size());
-      DCHECK(child_type->Equals(child_data[column_idx]->type));
-      chunks[chunk_idx] = child_data[column_idx];
+      const auto& parent = chunk(chunk_idx);
+      const auto& children = parent->child_data;
+      DCHECK_EQ(columns.size(), children.size());
+      auto child = children[column_idx]->Slice(parent->offset, parent->length);

Review Comment:
   Yes (a bug that I introduced in https://github.com/apache/arrow/pull/34537). It gives incorrect results when retrieving a nested child from an array sliced from another array.
   
   Although, now that I'm thinking about it, I'd imagine the same problem would occur for the non-chunked `FieldPath::Get` variants as well. I'll need to test it and create a separate issue.



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