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https://issues.apache.org/jira/browse/HIVE-105?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12654012#action_12654012
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Joydeep Sen Sarma commented on HIVE-105:
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don't understand why we are capping at mapred.reduce.tasks? what's the
intention here? If we are introducing a new parameter to set a max reducer
count for all hadoop jobs - then i would argue that such a control needs to be
provided as part of hadoop administration options - not hive.
will the explain plan be enhanced to show input size? (at least for the root
tasks?)
for the per-query variable - we should change HiveConf to annotate some
variables as 'per-query' - and perhaps show it categorized such (somehow) when
the user displays all the variables
> 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
>
> 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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