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

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

    [ https://issues.apache.org/jira/browse/ARROW-3978?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17360064#comment-17360064 ] 

Wes McKinney commented on ARROW-3978:
-------------------------------------

[~bkietz] [~michalno] seems like this could be supported pretty soon using the new hash table machinery? 

> [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
>            Priority: Major
>              Labels: query-engine
>
> 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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