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

• Type: Improvement
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 8.0.0
• Component/s: C++
• Labels: good-second-issue kernel pull-request-available
• External issue URL: https://github.com/apache/arrow/issues/29593
• Assignee: Dhruv Vats
• Reporter: Ian Cook
• Created: 14/Sep/21 13:53
• Updated: 11/Jan/23 08:36
• Resolved: 17/Feb/22 13:13

Description

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

SELECT country, country_abbr FROM table GROUP BY country, country_abbr

but it would be more efficient to do

SELECT country, first(country_abbr) FROM table GROUP BY country

because then the engine does not need to scan all the values of the country_abbr column.

Issue Links

• blocks: ARROW-14045 [R] Support for .keep_all = TRUE with distinct()
• is related to: ARROW-15717 [Docs] Add hash_one to the documentation
• links to: GitHub Pull Request #12368