estimate number of required reducers and other map-reduce parameters
automatically
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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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