[ARROW-9132] [C++] Implement hash kernels for dictionary data with constant dictionaries

• Type: Improvement
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 0.17.1
• Fix Version/s: 1.0.0
• Component/s: C++, Python
• Labels: pull-request-available
• External issue URL: https://github.com/apache/arrow/issues/25243
• Assignee: Wes McKinney
• Reporter: Dave Hirschfeld
• Created: 15/Jun/20 12:02
• Updated: 11/Jan/23 08:04
• Resolved: 27/Jun/20 16:12

Description

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:

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.

Issue Links

• links to: GitHub Pull Request #7551