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From "Namit Jain (JIRA)" <>
Subject [jira] Commented: (HIVE-105) estimate number of required reducers and other map-reduce parameters automatically
Date Wed, 28 Jan 2009 19:31:03 GMT


Namit Jain commented on HIVE-105:

This change breaks the current behavior for group-by:

For the second stage, the number of reducers are inferred currently irrespective of what the
user sets the number of reducers to.

However, if the number of reducers are set, it will be carried over to the 2nd stage also.

Since, number of reducers is a function of data size by default now, we can get rid of it
in explain plan

> estimate number of required reducers and other map-reduce parameters automatically
> ----------------------------------------------------------------------------------
>                 Key: HIVE-105
>                 URL:
>             Project: Hadoop Hive
>          Issue Type: Improvement
>          Components: Query Processor
>            Reporter: Joydeep Sen Sarma
>            Assignee: Zheng Shao
>         Attachments: HIVE-105.1.patch, HIVE-105.2.patch
> 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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