[I] [C++] Sliced sparse union element access can return incorrect values [arrow]

["fenfeng9 (via GitHub)" <gi...@apache.org>]

fenfeng9 opened a new issue, #50105:
URL: https://github.com/apache/arrow/issues/50105

   ### Describe the bug, including details regarding any error messages, version, and platform.
   
   ### Description
   Slicing a sparse union array produces incorrect logical values.
   `to_pylist()` and scalar access on the slice return wrong results —a non-null element is reported as null, or a value from the wrong position is returned.
   ### Reproduce
   ```python
   import pyarrow as pa
   
   
   def main():
       arr = pa.UnionArray.from_sparse(
           pa.array([0, 0, 0], type=pa.int8()),
           [pa.array([None, 20, 30])],
       )
       sliced = arr.slice(2)
   
       print(f"pyarrow: {pa.__version__}")
       print()
   
       # Direct access on the original array is still correct.
       expected_arr_2 = 30
       actual_arr_2 = arr[2].as_py()
       print(f"arr[2].as_py() expected: {expected_arr_2}")
       print(f"arr[2].as_py() actual:   {actual_arr_2}")
       print()
   
       # The sliced array should preserve the same logical value in to_pylist().
       expected_sliced_pylist = [30]
       actual_sliced_pylist = sliced.to_pylist()
       print(f"sliced.to_pylist() expected: {expected_sliced_pylist}")
       print(f"sliced.to_pylist() actual:   {actual_sliced_pylist}")
       print()
   
       # Scalar access on the sliced array should also return the same value.
       expected_sliced_0 = 30
       actual_sliced_0 = sliced[0].as_py()
       print(f"sliced[0].as_py() expected: {expected_sliced_0}")
       print(f"sliced[0].as_py() actual:   {actual_sliced_0}")
   
   
   if __name__ == "__main__":
       main()
   ```
   ### Result
   ```python
   pyarrow: 24.0.0
   
   arr[2].as_py() expected: 30
   arr[2].as_py() actual:   30
   
   sliced.to_pylist() expected: [30]
   sliced.to_pylist() actual:   [None]
   
   sliced[0].as_py() expected: 30
   sliced[0].as_py() actual:   None
   ```
   
   ### Component(s)
   
   C++


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Re: [I] [C++] Sliced sparse union element access can return incorrect values [arrow]

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

pitrou commented on issue #50105:
URL: https://github.com/apache/arrow/issues/50105#issuecomment-4671504370

   Issue resolved by pull request 50108
   https://github.com/apache/arrow/pull/50108


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Re: [I] [C++] Sliced sparse union element access can return incorrect values [arrow]

["fenfeng9 (via GitHub)" <gi...@apache.org>]

fenfeng9 commented on issue #50105:
URL: https://github.com/apache/arrow/issues/50105#issuecomment-4632287224

   I think this bug is in the sparse-union logical null path here:
   
   https://github.com/apache/arrow/blob/e16067a78ce4e7325a258bb2ae1ef73211cc7d46/cpp/src/arrow/array/data.cc#L72-L77
   
   and here:
   
   https://github.com/apache/arrow/blob/e16067a78ce4e7325a258bb2ae1ef73211cc7d46/cpp/src/arrow/array/data.cc#L697-L702
   
   `IsNullSparseUnion` checks child nullness with `i`, but it should use `data.offset + i`. Likewise, `ArraySpan::IsNullSparseUnion` should use `this->offset + i` instead of `i`. Otherwise, sliced sparse unions can return incorrect logical values.
   
   I have verified the fix locally and will open a PR.


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Re: [I] [C++] Sliced sparse union element access can return incorrect values [arrow]

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

pitrou closed issue #50105: [C++] Sliced sparse union element access can return incorrect values
URL: https://github.com/apache/arrow/issues/50105


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