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

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

westonpace commented on code in PR #34270:
URL: https://github.com/apache/arrow/pull/34270#discussion_r1123860948


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cpp/src/arrow/compute/kernels/scalar_cast_temporal.cc:
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@@ -452,6 +459,16 @@ void AddCrossUnitCast(CastFunction* func) {
   DCHECK_OK(func->AddKernel(Type::type_id, std::move(kernel)));
 }
 
+template <typename Type>
+void AddCrossUnitCastNoPreallocate(CastFunction* func) {
+  ScalarKernel kernel;
+  kernel.exec = CastFunctor<Type, Type>::Exec;
+  kernel.null_handling = NullHandling::INTERSECTION;
+  kernel.mem_allocation = MemAllocation::NO_PREALLOCATE;

Review Comment:
   ```suggestion
     // We cannot preallocate in this case because some of these casts may be zero
     // copy if only the timezone (and not the units) change.  There is no way to distinguish
     // between these two cases based on input matching so we disable preallocation for
     // all of them.
     kernel.mem_allocation = MemAllocation::NO_PREALLOCATE;
   ```



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