[jira] [Resolved] (ARROW-12728) [C++][Compute] Aggregates: implement count distinct

["Ben Kietzman (Jira)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/ARROW-12728?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Ben Kietzman resolved ARROW-12728.
----------------------------------
    Resolution: Fixed

Issue resolved by pull request 10876
[https://github.com/apache/arrow/pull/10876]

> [C++][Compute] Aggregates: implement count distinct
> ---------------------------------------------------
>
>                 Key: ARROW-12728
>                 URL: https://issues.apache.org/jira/browse/ARROW-12728
>             Project: Apache Arrow
>          Issue Type: Improvement
>          Components: C++
>    Affects Versions: 4.0.0
>            Reporter: Michal Nowakiewicz
>            Assignee: David Li
>            Priority: Major
>              Labels: kernel, pull-request-available
>             Fix For: 6.0.0
>
>          Time Spent: 2h 40m
>  Remaining Estimate: 0h
>
> Implement count distinct aggregate reusing hash table from hash group by inside of it.
> This brings support to SQL queries like:
> select a, count(distinct b), count(distinct c) from t group by a
> For instance to compute count(distinct b), the first group id mapping will give group id based on column a value; then the second group id mapping is done using the key (groupid(a), b) inside count(distinct b) aggregate (similarly for count(distinct c)). 
> After all input rows are consumed, the final processing step scans the hash tables based on (groupid(a), b) and updates an array of counts indexed by groupid(a). 
> The resulting array of counts represents the output of count distinct aggregate.



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