[ARROW-11673] [C++] Casting dictionary type to use different index type

• Type: Improvement
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 5.0.0
• Component/s: C++
• Labels: pull-request-available
• External issue URL: https://github.com/apache/arrow/issues/27536
• Assignee: Niranda Perera
• Reporter: Joris Van den Bossche
• Created: 17/Feb/21 16:56
• Updated: 11/Jan/23 08:21
• Resolved: 20/Jul/21 16:40

Description

It's currently not implemented to cast from one dictionary type to another dictionary type to change the index type.

Small example:

In [2]: arr = pa.array(['a', 'b', 'a']).dictionary_encode()

In [3]: arr.type
Out[3]: DictionaryType(dictionary<values=string, indices=int32, ordered=0>)

In [5]: arr.cast(pa.dictionary(pa.int8(), pa.string()))
...
ArrowNotImplementedError: Unsupported cast from dictionary<values=string, indices=int32, ordered=0> to dictionary<values=string, indices=int8, ordered=0> (no available cast function for target type)
../src/arrow/compute/cast.cc:112  GetCastFunctionInternal(cast_options->to_type, args[0].type().get())

From https://stackoverflow.com/questions/66223730/how-to-change-column-datatype-with-pyarrow

Issue Links

• is cloned by: ARROW-13102 [Python] Casting between dictionary types
• links to: GitHub Pull Request #10330
• links to: GitHub Pull Request #10721