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https://issues.apache.org/jira/browse/HIVE-3952?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13641375#comment-13641375
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Namit Jain commented on HIVE-3952:
----------------------------------
https://reviews.facebook.net/D10533 just for the record
> merge map-job followed by map-reduce job
> ----------------------------------------
>
> Key: HIVE-3952
> URL: https://issues.apache.org/jira/browse/HIVE-3952
> Project: Hive
> Issue Type: Improvement
> Components: Query Processor
> Reporter: Namit Jain
> Assignee: Vinod Kumar Vavilapalli
> Attachments: hive.3952.1.patch, HIVE-3952-20130226.txt,
> HIVE-3952-20130227.1.txt, HIVE-3952-20130301.txt, HIVE-3952-20130421.txt,
> HIVE-3952-20130424.txt
>
>
> Consider the query like:
> select count(*) FROM
> ( select idOne, idTwo, value FROM
> bigTable
> JOIN
>
> smallTableOne on (bigTable.idOne = smallTableOne.idOne)
>
> ) firstjoin
>
> JOIN
>
> smallTableTwo on (firstjoin.idTwo = smallTableTwo.idTwo);
> where smallTableOne and smallTableTwo are smaller than
> hive.auto.convert.join.noconditionaltask.size and
> hive.auto.convert.join.noconditionaltask is set to true.
> The joins are collapsed into mapjoins, and it leads to a map-only job
> (for the map-joins) followed by a map-reduce job (for the group by).
> Ideally, the map-only job should be merged with the following map-reduce job.
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