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