[jira] [Updated] (ARROW-12725) [C++][Compute] GroupBy: improve performance by encoding keys in row format only when they are inserted into hash table

["ASF GitHub Bot (Jira)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/ARROW-12725?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

ASF GitHub Bot updated ARROW-12725:
-----------------------------------
    Labels: pull-request-available  (was: )

> [C++][Compute] GroupBy: improve performance by encoding keys in row format only when they are inserted into hash table
> ----------------------------------------------------------------------------------------------------------------------
>
>                 Key: ARROW-12725
>                 URL: https://issues.apache.org/jira/browse/ARROW-12725
>             Project: Apache Arrow
>          Issue Type: Improvement
>          Components: C++
>    Affects Versions: 4.0.0
>            Reporter: Michal Nowakiewicz
>            Priority: Major
>              Labels: pull-request-available
>             Fix For: 5.0.0
>
>          Time Spent: 10m
>  Remaining Estimate: 0h
>
> Previous implementation of hash group by converts input ExecBatches to row-oriented format,
> then hashes and compares rows as if they were a single column. 
> It is more efficient (especially for small number of key columns) to avoid relatively costly 
> encoding and instead compute hashes of individual columns in column-oriented format mixing them together, and similarly comparing column-oriented data to row-oriented data in the hash table without converting. 
> Encoding only happens for a subset of input rows that are inserted into the hash table - they introduce new groups. 
> Keys in hash table remain stored as row-oriented.



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