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[jira] [Created] (ARROW-3978) [C++] Implement hashing, dictionary-encoding for StructArray


Wes McKinney created ARROW-3978:
-----------------------------------

             Summary: [C++] Implement hashing, dictionary-encoding for StructArray
                 Key: ARROW-3978
                 URL: https://issues.apache.org/jira/browse/ARROW-3978
             Project: Apache Arrow
          Issue Type: New Feature
          Components: C++
            Reporter: Wes McKinney
             Fix For: 0.13.0


This is a central requirement for hash-aggregations such as

{code}
SELECT AGG_FUNCTION(expr)
FROM table
GROUP BY expr1, expr2, ...
{code}

The materialized keys in the GROUP BY section form a struct, which can be incrementally hashed to produce dictionary codes suitable for computing aggregates or any other purpose. 

There are a few subtasks related to this, such as efficiently constructing a record (that can be hashed quickly) to identify each "row" in the struct. Maybe we should start with that first



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