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https://issues.apache.org/jira/browse/HIVE-2566?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13150013#comment-13150013
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He Yongqiang commented on HIVE-2566:
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looks good, running tests.
> reduce the number map-reduce jobs for union all
> -----------------------------------------------
>
> Key: HIVE-2566
> URL: https://issues.apache.org/jira/browse/HIVE-2566
> Project: Hive
> Issue Type: Improvement
> Reporter: Namit Jain
> Assignee: Namit Jain
> Attachments: HIVE-2566.D405.1.patch, HIVE-2566.D405.2.patch
>
>
> A query like:
> select s.key, s.value from (
> select key, value from src2 where key < 10
> union all
> select key, value from src3 where key < 10
> union all
> select key, value from src4 where key < 10
> union all
> select key, count(1) as value from src5 group by key
> )s;
> should run the last sub-query
> 'select key, count(1) as value from src5 group by key'
> as a map-reduce job.
> And then the union should be a map-only job reading from the first 3 map-only
> subqueries
> and the output of the last map-reduce job.
> The current plan is very inefficient.
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