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https://issues.apache.org/jira/browse/HIVE-2566?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13174520#comment-13174520
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Hudson commented on HIVE-2566:
------------------------------
Integrated in Hive-trunk-h0.21 #1164 (See
[https://builds.apache.org/job/Hive-trunk-h0.21/1164/])
HIVE-2642 fix Hive-2566 and make union optimization more aggressive
(Yongqiang He via namit)
> 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
> Fix For: 0.8.0
>
> Attachments: HIVE-2566.D405.1.patch, HIVE-2566.D405.2.patch,
> HIVE-2566.D405.3.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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