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From "Aaron Kimball (JIRA)" <j...@apache.org>
Subject [jira] Created: (HADOOP-5967) Sqoop should only use a single map task
Date Thu, 04 Jun 2009 01:04:07 GMT
Sqoop should only use a single map task
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                 Key: HADOOP-5967
                 URL: https://issues.apache.org/jira/browse/HADOOP-5967
             Project: Hadoop Core
          Issue Type: Improvement
            Reporter: Aaron Kimball
            Assignee: Aaron Kimball
            Priority: Minor
         Attachments: single-mapper.patch

The current DBInputFormat implementation uses SELECT ... LIMIT ... OFFSET statements to read
from a database table. This actually results in several queries all accessing the same table
at the same time. Most database implementations will actually use a full table scan for each
such query, starting at row 1 and scanning down until the OFFSET is reached before emitting
data to the client. The upshot of this is that we see O(n^2) performance in the size of the
table when using a large number of mappers, when a single mapper would read through the table
in O(n) time in the number of rows.

This patch sets the number of map tasks to 1 in the MapReduce job sqoop launches.

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