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https://issues.apache.org/jira/browse/ARROW-13993?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17488089#comment-17488089
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Dhruv Vats commented on ARROW-13993:
------------------------------------
Just so I understand this correctly (as I don't have a very formal CS
background), when we do:
{code:sql}
SELECT country, SUM(customerID) FROM db_table GROUP BY country{code}
from a supposed sales table {{db_table}} that has fields {{country}} and
{{{}customerID{}}}, we get number of customers _per_ country/group.
So here instead sum of all tuples in a group, we just want to return a single
tuple from the different groups/country? And, it seems _which_ tuple (like
either the first or a specific one) to return is yet to be finalised, right?
Also is there a PR or an existing kernel that has a similar boilerplate code to
what this will have? (That'll save a disproportionate time going through all
the abstractions).
> [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
> Fix For: 8.0.0
>
>
> 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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