[jira] [Comment Edited] (ARROW-13993) [C++] Hash aggregate function that returns value from first row in group

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

    [ https://issues.apache.org/jira/browse/ARROW-13993?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17488095#comment-17488095 ] 

David Li edited comment on ARROW-13993 at 2/7/22, 1:17 PM:
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Yes, we just want a single row per group. Any row will do; the point above is that we can't implement anything else (because the query engine currently lacks support for ordering, beyond sorting outputs at the very end).

All hash_ kernels ("hash aggregate kernels") are in [{{hash_aggregate.cc}}|https://github.com/apache/arrow/blob/master/cpp/src/arrow/compute/kernels/hash_aggregate.cc] and it will be very similar to the CountDistinct/Distinct implementation there. 


was (Author: lidavidm):
Yes, we just want a single tuple. Any tuple will do; the point above is that we can't implement anything else (because the query engine currently lacks support for ordering, beyond sorting outputs at the very end).

All hash_ kernels ("hash aggregate kernels") are in [{{hash_aggregate.cc}}|https://github.com/apache/arrow/blob/master/cpp/src/arrow/compute/kernels/hash_aggregate.cc] and it will be very similar to the CountDistinct/Distinct implementation there. 

> [C++] Hash aggregate function that returns value from first row in group
> ------------------------------------------------------------------------
>
>                 Key: ARROW-13993
>                 URL: https://issues.apache.org/jira/browse/ARROW-13993
>             Project: Apache Arrow
>          Issue Type: Improvement
>          Components: C++
>            Reporter: Ian Cook
>            Assignee: Dhruv Vats
>            Priority: Major
>              Labels: good-second-issue, kernel
>             Fix For: 8.0.0
>
>
> It would be nice to have a hash aggregate function that returns the first value of a column within each hash group.
> If row order within groups is non-deterministic, then effectively this would return one arbitrary value. This is a very computationally cheap operation.
> This can be quite useful when querying a non-normalized table. For example if you have a table with a {{country}} column and also a {{country_abbr}} column and you want to group by either/both of those columns but return the values from both columns, you could do
> {code:java}
> SELECT country, country_abbr FROM table GROUP BY country, country_abbr{code}
> but it would be more efficient to do
> {code:java}
> SELECT country, first(country_abbr) FROM table GROUP BY country{code}
> because then the engine does not need to scan all the values of the {{country_abbr}} column.



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