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https://issues.apache.org/jira/browse/DRILL-4147?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17555188#comment-17555188
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ASF GitHub Bot commented on DRILL-4147:
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hsuanyi closed pull request #288: DRILL-4147: Change UnionPrel's
DrillDistributionTrait to be ANY to al…
URL: https://github.com/apache/drill/pull/288
> Union All operator runs in a single fragment
> --------------------------------------------
>
> Key: DRILL-4147
> URL: https://issues.apache.org/jira/browse/DRILL-4147
> Project: Apache Drill
> Issue Type: Bug
> Reporter: amit hadke
> Assignee: Aman Sinha
> Priority: Major
> Fix For: 1.8.0
>
>
> A User noticed that running select from a single directory is much faster
> than union all on two directories.
> (https://drill.apache.org/blog/2014/12/09/running-sql-queries-on-amazon-s3/#comment-2349732267)
>
> It seems like UNION ALL operator doesn't parallelize sub scans (its using
> SINGLETON for distribution type). Everything is ran in single fragment.
> We may have to use SubsetTransformer in UnionAllPrule.
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