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

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

Ian Cook created ARROW-13993:
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             Summary: [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


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 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, 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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