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https://issues.apache.org/jira/browse/ARROW-9147?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17256193#comment-17256193
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Gabriel Bassett commented on ARROW-9147:
----------------------------------------
I received the following error with arrow 2.0.0 (R):
```
Error in dataset___Scanner__ToTable(self): Type error: fields had matching
names but differing types. From: <column_name>: bool To: <column_name>: null
```
Should this be fixed? Is it possible the order in which the null is encountered
is handled differently?
> [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: Sub-task
> Components: C++
> Reporter: Joris Van den Bossche
> Assignee: Ben Kietzman
> Priority: Major
> Labels: dataset, dataset-dask-integration, pull-request-available
> Fix For: 2.0.0
>
> Time Spent: 1h 40m
> Remaining Estimate: 0h
>
> 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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