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Date: Wed, 3 Jun 2009 18:04:07 -0700 (PDT)
From: "Aaron Kimball (JIRA)" <jira@apache.org>
To: core-dev@hadoop.apache.org
Subject: [jira] Created: (HADOOP-5967) Sqoop should only use a single map
 task
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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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