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From "Yin Huai (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HIVE-5258) Optimize aggregations without Group By followed by a Cross Join
Date Tue, 10 Sep 2013 15:42:53 GMT

    [ https://issues.apache.org/jira/browse/HIVE-5258?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13763129#comment-13763129
] 

Yin Huai commented on HIVE-5258:
--------------------------------

Handling aggregations with the DISTINCT keyword will be tricky. We may overwhelm the single
reducer when those columns with the DISTINCT keyword have a lot of distinct values. 
                
> Optimize aggregations without Group By followed by a Cross Join
> ---------------------------------------------------------------
>
>                 Key: HIVE-5258
>                 URL: https://issues.apache.org/jira/browse/HIVE-5258
>             Project: Hive
>          Issue Type: Improvement
>            Reporter: Yin Huai
>            Assignee: Yin Huai
>
> For example, we should use a single MR job to execute the following query
> {code:sql}
> SELECT *
> FROM (SELECT tmp1.cnt1, tmp2.cnt2
>       FROM (SELECT count(*) as cnt1
>             FROM src1 x) tmp1
>       JOIN (SELECT count(*) as cnt2
>             FROM src1 y) tmp2) tmp3;
> {code}
> The reduce phase should have the reduce side GroupByOperators of tmp1 and tmp2, and the
JoinOperator for the cross join.

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