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https://issues.apache.org/jira/browse/HIVE-105?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12665654#action_12665654
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Zheng Shao commented on HIVE-105:
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> most hadoop installations have a non-zero value specified for this.
> hadoop-default.xml comes with a value of 1. Did u mean if it is > 1?
User would need to override this value in the session or hive-default/site.xml.
The reason for this is just for backward compatibility (it adds potential
confusions because both hive and hadoop are reading this "mapred.reduce.tasks"
and hive might potentially change it in case of reducer number estimation).
Otherwise it's much cleaner to use a separate variable (like
"hive.exec.reduce.tasks"), with the cost of backward compatibility.
> estimate number of required reducers and other map-reduce parameters
> automatically
> ----------------------------------------------------------------------------------
>
> Key: HIVE-105
> URL: https://issues.apache.org/jira/browse/HIVE-105
> Project: Hadoop Hive
> Issue Type: Improvement
> Components: Query Processor
> Reporter: Joydeep Sen Sarma
> Assignee: Zheng Shao
>
> currently users have to specify number of reducers. In a multi-user
> environment - we generally ask users to be prudent in selecting number of
> reducers (since they are long running and block other users). Also - large
> number of reducers produce large number of output files - which puts pressure
> on namenode resources.
> there are other map-reduce parameters - for example the min split size and
> the proposed use of combinefileinputformat that are also fairly tricky for
> the user to determine (since they depend on map side selectivity and cluster
> size). This will become totally critical when there is integration with BI
> tools since there will be no opportunity to optimize job settings and there
> will be a wide variety of jobs.
> This jira calls for automating the selection of such parameters - possibly by
> a best effort at estimating map side selectivity/output size using sampling
> and determining such parameters from there.
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