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

["David Li (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/ARROW-12728?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17380712#comment-17380712 ] 

David Li commented on ARROW-12728:
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If I understand right, this should be a fairly straightfoward hash aggregate kernel on top of ARROW-12759 - is anyone planning on working on this? I'd like to take it otherwise to get more familiar with the ExecNode/Grouper infrastructure.

> [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
>            Priority: Major
>             Fix For: 6.0.0
>
>
> 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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