[GitHub] [arrow] rok commented on a diff in pull request #34270: GH-34210: [C++] Unexpected performance when casting from tz-naive to tz-aware timestamps

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

rok commented on code in PR #34270:
URL: https://github.com/apache/arrow/pull/34270#discussion_r1113666743


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cpp/src/arrow/compute/kernels/scalar_cast_temporal.cc:
##########
@@ -153,8 +155,12 @@ struct CastFunctor<
     const auto& in_type = checked_cast<const I&>(*batch[0].type());
     const auto& out_type = checked_cast<const O&>(*output->type);
 
-    // The units may be equal if the time zones are different. We might go to
-    // lengths to make this zero copy in the future but we leave it for now
+    if (I::type_id == Type::TIMESTAMP && in_type.unit() == out_type.unit()) {
+      std::shared_ptr<const ArrayData> array_data = input.ToArrayData();
+      output->SetMembers(*array_data);
+      arrow::compute::detail::PropagateNullsSpans(batch, output);

Review Comment:
   I assumed (hoped) this would zero-copy the null bitmap. I suppose things are a little bit more subtle and I'll go read up a little bit.



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