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https://issues.apache.org/jira/browse/PIG-2652?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13254251#comment-13254251
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Dmitriy V. Ryaboy commented on PIG-2652:
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Figured it out.
The parallelism from MRCompiler is used to set the number of quantiles that
FindQuantiles needs to produce. If we set it to -1, nothing gets generated. We
need to somehow do the proper, stat-guided estimation prior to getSamplingJob
getting called (or fix it up in JobControlCompiler).
> Skew join and order by don't trigger reducer estimation
> -------------------------------------------------------
>
> Key: PIG-2652
> URL: https://issues.apache.org/jira/browse/PIG-2652
> Project: Pig
> Issue Type: Bug
> Reporter: Bill Graham
> Assignee: Bill Graham
> Fix For: 0.10.0, 0.9.3, 0.11
>
> Attachments: PIG-2652_1.patch
>
>
> If neither PARALLEL, default parallel or {{mapred.reduce.tasks}} are set, the
> number of reducers is not estimated based on input size for skew joins or
> order by. Instead, these jobs get only 1 reducer.
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