[GitHub] [arrow] jorisvandenbossche commented on pull request #35522: GH-35521: [C++] Hash null bitmap only if null count is 0

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

jorisvandenbossche commented on PR #35522:
URL: https://github.com/apache/arrow/pull/35522#issuecomment-1542036825

   @pitrou small example using python (you don't actually need to filter or slice yourself, since a scalar is already a slice into its parent array):
   
   ```
   In [31]: arr1 = pa.array([[0, 1], [2, 3]])
   
   In [32]: scalar1 = arr1[0]
   
   In [33]: arr2 = pa.array([[0, 1], [2, None]])
   
   In [34]: scalar2 = arr2[0]
   
   In [35]: scalar1
   Out[35]: <pyarrow.ListScalar: [0, 1]>
   
   In [36]: scalar2
   Out[36]: <pyarrow.ListScalar: [0, 1]>
   
   In [37]: hash(scalar1)
   Out[37]: 6972737373264176731
   
   In [38]: hash(scalar2)
   Out[38]: 5286180417804377197
   
   In [39]: scalar1.values.buffers()
   Out[39]: 
   [None,
    <pyarrow.Buffer address=0x7fbc56a08200 size=32 is_cpu=True is_mutable=True>]
   
   In [40]: scalar2.values.buffers()
   Out[40]: 
   [<pyarrow.Buffer address=0x7fbc56a08180 size=1 is_cpu=True is_mutable=True>,
    <pyarrow.Buffer address=0x7fbc56a08280 size=32 is_cpu=True is_mutable=True>]
   ```
   
   Those two scalars are equal, so should have the same hash. But the one has a validity bitmap, and the other not.


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