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https://issues.apache.org/jira/browse/ARROW-3978?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16853069#comment-16853069
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Francois Saint-Jacques commented on ARROW-3978:
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ClickHouse also uses the pivot method, see
*
[https://github.com/yandex/ClickHouse/blob/d4f474cd196c1b7dff65f8507e87b380f64e2b53/dbms/src/Common/ColumnsHashing.h#L513-L540]
*
https://github.com/yandex/ClickHouse/blob/d4f474cd196c1b7dff65f8507e87b380f64e2b53/dbms/src/Interpreters/AggregationCommon.h#L231-L243
*
https://github.com/yandex/ClickHouse/blob/d4f474cd196c1b7dff65f8507e87b380f64e2b53/dbms/src/Columns/ColumnVector.cpp#L32-L45
> [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
> Fix For: 0.14.0
>
>
> 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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