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https://issues.apache.org/jira/browse/HADOOP-5967?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12716186#action_12716186
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Aaron Kimball commented on HADOOP-5967:
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An ORDER BY clause is included in DBInputFormat's SQL statements that it sends
over JDBC. But each mapper (necessarily) runs in a separate transaction, as
it's on a separate node.
> Sqoop should only use a single map task
> ---------------------------------------
>
> 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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