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https://issues.apache.org/jira/browse/HIVE-3972?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13569944#comment-13569944
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Navis commented on HIVE-3972:
-----------------------------
Top-K optimization is already on HIVE-3562 and seemed orthogonal with this
issue. And top-k will make this further less useful.
The reason why I made this is that there are so many statements like 'hive is
quite inefficient handling order by cause its ran by single reducer' and I just
hate hear that. This is not important issue but can be a start point for other
optimization exploiting ordered traits from multiple bucket files.
Added configuration means number of reducer for lastly order-by MR stage. -1
means it will be decided by usual calculation. 0 means disabling this.
> Support using multiple reducer for fetching order by results
> ------------------------------------------------------------
>
> Key: HIVE-3972
> URL: https://issues.apache.org/jira/browse/HIVE-3972
> Project: Hive
> Issue Type: Improvement
> Components: Query Processor
> Reporter: Navis
> Assignee: Navis
> Priority: Minor
> Attachments: HIVE-3972.D8349.1.patch
>
>
> Queries for fetching results which have lastly "order by" clause make final
> MR run with single reducer, which can be too much. For example,
> {code}
> select value, sum(key) as sum from src group by value order by sum;
> {code}
> If number of reducer is reasonable, multiple result files could be merged
> into single sorted stream in the fetcher level.
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