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https://issues.apache.org/jira/browse/ARROW-12420?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17323792#comment-17323792
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Uwe Korn commented on ARROW-12420:
----------------------------------
cc [~bkietz] who wrote the PR that broke it ;)
> [C++/Dataset] Reading null columns as dictionary not longer possible
> --------------------------------------------------------------------
>
> Key: ARROW-12420
> URL: https://issues.apache.org/jira/browse/ARROW-12420
> Project: Apache Arrow
> Issue Type: Improvement
> Components: C++
> Affects Versions: 4.0.0
> Reporter: Uwe Korn
> Priority: Major
> Fix For: 4.0.0
>
>
> Reading a dataset with a dictionary column where some of the files don't
> contain any data for that column (and thus are typed as null) broke with
> https://github.com/apache/arrow/pull/9532. It worked with the 3.0 release
> though and thus I would consider this a regression.
> This can be reproduced using the following Python snippet:
> {code}
> import pyarrow as pa
> import pyarrow.parquet as pq
> import pyarrow.dataset as ds
> table = pa.table({"a": [None, None]})
> pq.write_table(table, "test.parquet")
> schema = pa.schema([pa.field("a", pa.dictionary(pa.int32(), pa.string()))])
> fsds = ds.FileSystemDataset.from_paths(
> paths=["test.parquet"],
> schema=schema,
> format=pa.dataset.ParquetFileFormat(),
> filesystem=pa.fs.LocalFileSystem(),
> )
> fsds.to_table()
> {code}
> The exception on master is currently:
> {code}
> ---------------------------------------------------------------------------
> ArrowNotImplementedError Traceback (most recent call last)
> <ipython-input-14-5f0bc602f16b> in <module>
> 6 filesystem=pa.fs.LocalFileSystem(),
> 7 )
> ----> 8 fsds.to_table()
> ~/Development/arrow/python/pyarrow/_dataset.pyx in
> pyarrow._dataset.Dataset.to_table()
> 456 table : Table instance
> 457 """
> --> 458 return self._scanner(**kwargs).to_table()
> 459
> 460 def head(self, int num_rows, **kwargs):
> ~/Development/arrow/python/pyarrow/_dataset.pyx in
> pyarrow._dataset.Scanner.to_table()
> 2887 result = self.scanner.ToTable()
> 2888
> -> 2889 return pyarrow_wrap_table(GetResultValue(result))
> 2890
> 2891 def take(self, object indices):
> ~/Development/arrow/python/pyarrow/error.pxi in
> pyarrow.lib.pyarrow_internal_check_status()
> 139 cdef api int pyarrow_internal_check_status(const CStatus& status) \
> 140 nogil except -1:
> --> 141 return check_status(status)
> 142
> 143
> ~/Development/arrow/python/pyarrow/error.pxi in pyarrow.lib.check_status()
> 116 raise ArrowKeyError(message)
> 117 elif status.IsNotImplemented():
> --> 118 raise ArrowNotImplementedError(message)
> 119 elif status.IsTypeError():
> 120 raise ArrowTypeError(message)
> ArrowNotImplementedError: Unsupported cast from null to
> dictionary<values=string, indices=int32, ordered=0> (no available cast
> function for target type)
> {code}
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