[
https://issues.apache.org/jira/browse/ARROW-13993?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17494025#comment-17494025
]
David Li commented on ARROW-13993:
----------------------------------
See ARROW-15717.
> [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, pull-request-available
> Fix For: 8.0.0
>
> Time Spent: 5h 40m
> Remaining Estimate: 0h
>
> 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.
--
This message was sent by Atlassian Jira
(v8.20.1#820001)
