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