[jira] [Created] (ARROW-9147) [C++][Dataset] Support null -> other type promotion in Dataset scanning

["Joris Van den Bossche (Jira)" <ji...@apache.org>]

Joris Van den Bossche created ARROW-9147:
--------------------------------------------

             Summary: [C++][Dataset] Support null -> other type promotion in Dataset scanning
                 Key: ARROW-9147
                 URL: https://issues.apache.org/jira/browse/ARROW-9147
             Project: Apache Arrow
          Issue Type: Improvement
          Components: C++
            Reporter: Joris Van den Bossche
             Fix For: 1.0.0


With regarding schema evolution / normalization, we support inserting nulls for a missing column or changing nullability, or normalizing column order, but we do not yet seem to support promotion of null type to any other type.

Small python example:

{code}
In [11]: df = pd.DataFrame({"col": np.array([None, None, None, None], dtype='object')})
    ...: df.to_parquet("test_filter_schema.parquet", engine="pyarrow")
    ...:
    ...: import pyarrow.dataset as ds
    ...: dataset = ds.dataset("test_filter_schema.parquet", format="parquet", schema=pa.schema([("col", pa.int64())]))
    ...: dataset.to_table()
...
~/scipy/repos/arrow/python/pyarrow/_dataset.pyx in pyarrow._dataset.Dataset.to_table()
~/scipy/repos/arrow/python/pyarrow/_dataset.pyx in pyarrow._dataset.Scanner.to_table()
~/scipy/repos/arrow/python/pyarrow/error.pxi in pyarrow.lib.pyarrow_internal_check_status()
~/scipy/repos/arrow/python/pyarrow/error.pxi in pyarrow.lib.check_status()
ArrowTypeError: fields had matching names but differing types. From: col: null To: col: int64
{code}



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