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https://issues.apache.org/jira/browse/ARROW-13993?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Ian Cook updated ARROW-13993:
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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
{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.
was:
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, 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.
> [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
> Priority: Major
>
> 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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