[jira] [Created] (ARROW-12564) [C++] Add compute kernel for extract keys / items from Map type data

["Joris Van den Bossche (Jira)" <ji...@apache.org>]

Joris Van den Bossche created ARROW-12564:
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             Summary: [C++] Add compute kernel for extract keys / items from Map type data
                 Key: ARROW-12564
                 URL: https://issues.apache.org/jira/browse/ARROW-12564
             Project: Apache Arrow
          Issue Type: Improvement
          Components: C++
            Reporter: Joris Van den Bossche


For a single array ({{MapArray}}) there are the {{.keys}} and {{.items}} attributes (in Python, corresponding to {{MapArray::keys()/items()}} in C++) to extract the keys or items as a new array. 

However, when eg having a Table column as a ChunkedArray, there is not easy way to get a new ChunkedArray of keys (except by iterating through the chunks in Python: {{keys = pa.chunked_array([chunk.keys for chunk in map_data.iterchunks()])}})

We could add compute kernels {{map_keys}}/{{map_items}} to do this for both array or chunked array.



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