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

["Ian Cook (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/ARROW-13993?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17414961#comment-17414961 ] 

Ian Cook commented on ARROW-13993:
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A more general solution would be to implement a {{hash_take}} hash aggregate function that takes a scalar integer argument {{n}} and returns the {{n}}th row from each hash group.

> [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
>            Priority: Major
>              Labels: kernel
>
> 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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