Mailing-List: contact core-dev-help@hadoop.apache.org; run by ezmlm
Precedence: bulk
Reply-To: core-dev@hadoop.apache.org
Message-ID: <863889561.1244101687448.JavaMail.jira@brutus>
Date: Thu, 4 Jun 2009 00:48:07 -0700 (PDT)
From: "Aaron Kimball (JIRA)" <jira@apache.org>
To: core-dev@hadoop.apache.org
Subject: [jira] Commented: (HADOOP-5967) Sqoop should only use a single map
 task
In-Reply-To: <1164005404.1244077447429.JavaMail.jira@brutus>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 7bit


    [ https://issues.apache.org/jira/browse/HADOOP-5967?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12716186#action_12716186 ] 

Aaron Kimball commented on HADOOP-5967:
---------------------------------------

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.

-- 
This message is automatically generated by JIRA.
-
You can reply to this email to add a comment to the issue online.