How to Extract a Child Vector from a Dictionary-Wrapped RowVector?

[yukkit]

I have a RowVector wrapped in a dictionary, and it may be nested several layers deep. I want to retrieve only one of its sub-vectors. Is there a good way to do this?

[yukkit]

I can use flattenVector to achieve my goal, but it triggers a memory copy, which is quite frustrating.

  auto row_vector = makeRowVector({"id", "value"},
                                  {makeFlatVector<int64_t>({0, 1, 2, 3, 4}),
                                   makeFlatVector<facebook::velox::StringView>({"zhao", "qian", "sun", "li", "zhou"})});

  auto indices = makeIndices(row_vector->size(), [](auto i) { return i % 2; });
  auto dict_vector = wrapInDictionary(indices, row_vector);

  facebook::velox::BaseVector::flattenVector(dict_vector);

  auto flattn_vector = dict_vector->as<facebook::velox::RowVector>();

  auto id_vector = flattn_vector->childAt(0);