[jira] [Created] (ARROW-255) Finalize Dictionary representation

["Julien Le Dem (JIRA)" <ji...@apache.org>]

Julien Le Dem created ARROW-255:
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             Summary: Finalize Dictionary representation
                 Key: ARROW-255
                 URL: https://issues.apache.org/jira/browse/ARROW-255
             Project: Apache Arrow
          Issue Type: Improvement
          Components: Format
            Reporter: Julien Le Dem


format/Messages.fbs mentions DictionaryBatches with an id but does not specify where they are referenced.

We should add a {{dictionary: long}} in Field that references the dictionary id:

Field: https://github.com/apache/arrow/blob/34e7f48cb71428c4d78cf00d8fdf0045532d6607/format/Message.fbs#L86

Dictionary id: https://github.com/apache/arrow/blob/34e7f48cb71428c4d78cf00d8fdf0045532d6607/format/Message.fbs#L165

We need a spec in format/Layout.md that describes the dictionary layout.
When dictionary encoded the value vector is an array of unsigned int32.
The dictionary vector is a Vector of the type of the value. indexed by their id in the dictionary.



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