[jira] [Created] (ARROW-3144) [C++] Better solution for cases where dictionaries are unknown at schema reconstruction time, or for delta dictionaries

["Wes McKinney (JIRA)" <ji...@apache.org>]

Wes McKinney created ARROW-3144:
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             Summary: [C++] Better solution for cases where dictionaries are unknown at schema reconstruction time, or for delta dictionaries
                 Key: ARROW-3144
                 URL: https://issues.apache.org/jira/browse/ARROW-3144
             Project: Apache Arrow
          Issue Type: Improvement
          Components: C++
            Reporter: Wes McKinney
             Fix For: 0.12.0


There are a couple of inter-related issues:

* Cases where a system might send the schema without the dictionaries, and the user wishes to reason about the schema and its types without knowing the dictionary values

* Dictionaries that are changing, e.g. using delta dictionary messages

{{arrow::DictionaryType}} has no "linkage" to any external object. I propose adding a "LinkedDictionaryType" or something similar (purely a C++ construct), which functionally would be a subclass of {{DictionaryType}}, which would allow a type to be created which will obtain its dictionary later through some kind of "Dictionary provider" interface. There is something similar in Java already. This would allow a dictionary to evolve via delta dictionaries, or for a dictionary to be retrieved later e.g. through an RPC or IPC layer



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