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https://issues.apache.org/jira/browse/PIG-2652?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13255405#comment-13255405
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Dmitriy V. Ryaboy commented on PIG-2652:
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I've verified that the same bug (undercounting the input when estimating
reducers) is in effect on trunk, when SampleOptimizer is able to estimate
reducers.
Starting to uncover quite a few issues in skewed join implementation.. for
example even if I explicitly set parallelism to 56, and have a half-dozen
unique values in the skewed relation, the output is only split across 16
reducers.
> 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, PIG-2652_2.patch, PIG-2652_3.patch,
> PIG-2652_3_10.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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