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https://issues.apache.org/jira/browse/ARROW-8088?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Joris Van den Bossche reassigned ARROW-8088:
--------------------------------------------
Assignee: Ben Kietzman (was: Joris Van den Bossche)
> [C++][Dataset] Partition columns with specified dictionary type result in all
> nulls
> -----------------------------------------------------------------------------------
>
> Key: ARROW-8088
> URL: https://issues.apache.org/jira/browse/ARROW-8088
> Project: Apache Arrow
> Issue Type: Bug
> Components: C++ - Dataset
> Reporter: Joris Van den Bossche
> Assignee: Ben Kietzman
> Priority: Major
> Labels: pull-request-available
> Fix For: 0.17.0
>
> Time Spent: 0.5h
> Remaining Estimate: 0h
>
> When specifying an explicit schema for the Partitioning, and when using a
> dictionary type, the materialization of the partition keys goes wrong: you
> don't get an error, but you get columns with all nulls.
> Python example:
> {code:python}
> foo_keys = [0, 1]
> bar_keys = ['a', 'b', 'c']
> N = 30
> df = pd.DataFrame({
> 'foo': np.array(foo_keys, dtype='i4').repeat(15),
> 'bar': np.tile(np.tile(np.array(bar_keys, dtype=object), 5), 2),
> 'values': np.random.randn(N)
> })
> pq.write_to_dataset(pa.table(df), "test_order", partition_cols=['foo', 'bar'])
> {code}
> When reading with discovery, all is fine:
> {code:python}
> >>> ds.dataset("test_order", format="parquet",
> >>> partitioning="hive").to_table().schema
> values: double
> bar: string
> foo: int32
> >>> ds.dataset("test_order", format="parquet",
> >>> partitioning="hive").to_table().to_pandas().head(2)
> values bar foo
> 0 2.505903 a 0
> 1 -1.760135 a 0
> {code}
> But when specifying the partition columns to be dictionary type with explicit
> {{HivePartitioning}}, you get no error but all null values:
> {code:python}
> >>> partitioning = ds.HivePartitioning(pa.schema([
> ... ("foo", pa.dictionary(pa.int32(), pa.int64())),
> ... ("bar", pa.dictionary(pa.int32(), pa.string()))
> ... ]))
> >>> ds.dataset("test_order", format="parquet",
> >>> partitioning=partitioning).to_table().schema
> values: double
> foo: dictionary<values=int64, indices=int32, ordered=0>
> bar: dictionary<values=string, indices=int32, ordered=0>
> >>> ds.dataset("test_order", format="parquet",
> >>> partitioning=partitioning).to_table().to_pandas().head(2)
> values foo bar
> 0 2.505903 NaN NaN
> 1 -1.760135 NaN NaN
> {code}
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