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https://issues.apache.org/jira/browse/PIG-951?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12756654#action_12756654
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Alan Gates commented on PIG-951:
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I'll be reviewing this patch.
> Reset parallelism to 1 for indexing job in MergeJoin
> ----------------------------------------------------
>
> Key: PIG-951
> URL: https://issues.apache.org/jira/browse/PIG-951
> Project: Pig
> Issue Type: Bug
> Components: impl
> Reporter: Ashutosh Chauhan
> Assignee: Ashutosh Chauhan
> Attachments: pig-951.patch
>
>
> After sampling one tuple from every block, one reducer is used to sort the
> index entries in reduce phase to produce sorted index to be used in actual
> join job. Thus, parallelism of index job should be explictly set to 1.
> Currently, its not.
> Currently, this is a non-issue, since we don't allow any blocking operators
> in pipeline before merge-join. However, later when we do allow blocking
> operators, then parallelism of indexing job will be that of preceding
> blocking operator. Even then, job will complete successfully because all
> tuple will go to only one reducer, because we are grouping on only one key
> "all". However, it will waste cluster resources by starting all the extra
> reducers which get no data and thus do nothing.
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