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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