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From "Olga Natkovich (JIRA)" <j...@apache.org>
Subject [jira] Created: (HADOOP-3594) Guaranteeing that combiner is called at least once
Date Wed, 18 Jun 2008 17:48:45 GMT
Guaranteeing that combiner is called at least once

                 Key: HADOOP-3594
                 URL: https://issues.apache.org/jira/browse/HADOOP-3594
             Project: Hadoop Core
          Issue Type: Bug
            Reporter: Olga Natkovich
             Fix For: 0.18.0

In 18, hadoop decides how many times to call combiner on both map and reduce sides. The possible
number is between 0 and N. 

While having multiple invocations can be useful, not invoking combiner at all can have serious
consequences for a range of functions called algebraic (http://classweb.gmu.edu/kersch/inft864/Readings/Shoshani/DataCube/DataCubeTechReport.pdf).
The main properties of such functions is that the intermediate and final computations are
different and that the first invokation transforms the data to a different form. A most common
example of this is AVERAGE function. While it is possible to workaround this issue by annotating
each tuple, it seems that it would be much easier and faster if hadoop always guaranteed at
least a single invocation.
Not having this guarantee will break all sorts of existing combiners.

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