[jira] [Created] (ARROW-9132) Support unique for dictionary type

["Dave Hirschfeld (Jira)" <ji...@apache.org>]

Dave Hirschfeld created ARROW-9132:
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             Summary: Support unique for dictionary type
                 Key: ARROW-9132
                 URL: https://issues.apache.org/jira/browse/ARROW-9132
             Project: Apache Arrow
          Issue Type: Improvement
          Components: Python
    Affects Versions: 0.17.1
            Reporter: Dave Hirschfeld


Enabling [`strings_as_dictionary`](https://turbodbc.readthedocs.io/en/latest/pages/advanced_usage.html?highlight=strings_as_dictionary#obtaining-apache-arrow-result-sets) in `turbodbc` returns a `ChunkedArray` of `dictionary` type (IIUC).

I'd like to enable this for better performance however it seems not all functionality is implemented for `dictionary` types? In particular, `unique` seems not to be implemented:
```python
In [40]: nmi.__class__.mro()
Out[40]: [pyarrow.lib.ChunkedArray, pyarrow.lib._PandasConvertible, object]

In [41]: nmi.type
Out[41]: DictionaryType(dictionary<values=string, indices=int32, ordered=0>)

In [42]: nmi.unique()
Traceback (most recent call last):

  File "<ipython-input-42-0fcb7893d5c4>", line 1, in <module>
    nmi.unique()

  File "pyarrow\table.pxi", line 307, in pyarrow.lib.ChunkedArray.unique

  File "pyarrow\error.pxi", line 106, in pyarrow.lib.check_status

ArrowNotImplementedError: unique not implemented for dictionary<values=string, indices=int32, ordered=0>
```

It would be very useful if the `dictionary` type supported all the usual operations.




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