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From "Zheng Shao (JIRA)" <j...@apache.org>
Subject [jira] Created: (HIVE-1118) Hive merge map files should have different bytes/mapper setting
Date Fri, 29 Jan 2010 05:33:36 GMT
Hive merge map files should have different bytes/mapper setting
---------------------------------------------------------------

                 Key: HIVE-1118
                 URL: https://issues.apache.org/jira/browse/HIVE-1118
             Project: Hadoop Hive
          Issue Type: Improvement
            Reporter: Zheng Shao


Currently, by default, we get one reducer for each 1GB of input data.
It's also true for the conditional merge job that will run if the average file size is smaller
than a threshold.

This actually makes those job very slow, because each reducer needs to consume 1GB of data.

Alternatively, we can just use that threshold to determine the number of reducers per job
(or introduce a new parameter).
Let's say the threshold is 1MB, then we only start the the merge job if the average file size
is less than 1MB, and the eventual result file size will be around 1MB (or another small number).

This will remove the extreme cases where we have thousands of empty files, but still make
normal jobs fast enough.


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