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https://issues.apache.org/jira/browse/PIG-2652?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13256871#comment-13256871
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Dmitriy V. Ryaboy commented on PIG-2652:
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Ok I have a much cleaner version of the patch, but it still suffers from a
critical flaw -- the sampler needs to know the number of buckets to get samples
for, and if it's not explicitly specified, and the SampleOptimizer stuff
doesn't kick in, it still gets a single bucket to "distribute" skewed keys
between.
I think we have to go ahead with the approach of always sampling for a large
number of buckets (10000?), and distributing them evenly in the partitioner.
Daniel -- comments? Will attach my work-in-progress patch.
> 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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