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https://issues.apache.org/jira/browse/HIVE-647?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12732750#action_12732750
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Bill Graham commented on HIVE-647:
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Thanks for you explanation re the first LIMIT query, that makes sense.
Re-verified that ORDER BY does not use only 1 reducer in my tests.
SELECT user, SUM(numRequests) AS num FROM MyTable GROUP BY user ORDER BY num
DESC;
> SORT BY with GROUP ignored without LIMIT
> ----------------------------------------
>
> Key: HIVE-647
> URL: https://issues.apache.org/jira/browse/HIVE-647
> Project: Hadoop Hive
> Issue Type: Bug
> Components: Query Processor
> Reporter: Bill Graham
>
> For queries with GROUP BY and SORT BY, the sort is not handled properly when
> a LIMIT is not supplied. If I run the following two queries, the first
> returns properly sorted results. The second does not.
> SELECT user, SUM(numRequests) AS num FROM MyTable GROUP BY user SORT BY num
> DESC LIMIT 50;
> SELECT user, SUM(numRequests) AS num FROM MyTable GROUP BY user SORT BY num
> DESC;
> Explain is different for the two queries as well. The first uses 3 M/R jobs
> and the second only uses 2, which might be part of the problem.
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