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https://issues.apache.org/jira/browse/HIVE-3276?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Namit Jain updated HIVE-3276:
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Attachment: hive.3276.9.patch
> optimize union sub-queries
> --------------------------
>
> Key: HIVE-3276
> URL: https://issues.apache.org/jira/browse/HIVE-3276
> Project: Hive
> Issue Type: Bug
> Reporter: Namit Jain
> Assignee: Namit Jain
> Attachments: HIVE-3276.1.patch, hive.3276.2.patch, hive.3276.3.patch,
> hive.3276.4.patch, hive.3276.5.patch, hive.3276.6.patch, hive.3276.7.patch,
> hive.3276.8.patch, hive.3276.9.patch
>
>
> It might be a good idea to optimize simple union queries containing
> map-reduce jobs in at least one of the sub-qeuries.
> For eg:
> a query like:
> insert overwrite table T1 partition P1
> select * from
> (
> subq1
> union all
> subq2
> ) u;
> today creates 3 map-reduce jobs, one for subq1, another for subq2 and
> the final one for the union.
> It might be a good idea to optimize this. Instead of creating the union
> task, it might be simpler to create a move task (or something like a move
> task), where the outputs of the two sub-queries will be moved to the final
> directory. This can easily extend to more than 2 sub-queries in the union.
> This is very useful if there is a select * followed by filesink after the
> union. This can be independently useful, and also be used to optimize the
> skewed joins https://cwiki.apache.org/Hive/skewed-join-optimization.html.
> If there is a select, filter between the union and the filesink, the select
> and the filter can be moved before the union, and the follow-up job can
> still be removed.
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