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

["Aldrin M (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/ARROW-3978?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17554759#comment-17554759 ] 

Aldrin M commented on ARROW-3978:
---------------------------------

I am picking up ARROW-8991 today. [~bkietz] [~apitrou] , I assume that this issue is mostly using new mechanisms and then "exposing an interface". I need to build up my understanding, but the "exposing an interface" portion may be addressed by ARROW-8991.

I just wanted to ping here and double check that it makes sense to take this while I work on ARROW-8991. I also wanted to check if there's anything in particular I should look at that hasn't already been mentioned in previous comments.

> [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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