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Date: Tue, 8 Sep 2009 23:41:57 -0700 (PDT)
From: "Hadoop QA (JIRA)" <jira@apache.org>
To: mapreduce-issues@hadoop.apache.org
Subject: [jira] Commented: (MAPREDUCE-885) More efficient SQL queries for
 DBInputFormat
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    [ https://issues.apache.org/jira/browse/MAPREDUCE-885?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12752937#action_12752937 ] 

Hadoop QA commented on MAPREDUCE-885:
-------------------------------------

-1 overall.  Here are the results of testing the latest attachment 
  http://issues.apache.org/jira/secure/attachment/12419012/MAPREDUCE-885.6.patch
  against trunk revision 812546.

    +1 @author.  The patch does not contain any @author tags.

    +1 tests included.  The patch appears to include 4 new or modified tests.

    +1 javadoc.  The javadoc tool did not generate any warning messages.

    +1 javac.  The applied patch does not increase the total number of javac compiler warnings.

    -1 findbugs.  The patch appears to introduce 1 new Findbugs warnings.

    +1 release audit.  The applied patch does not increase the total number of release audit warnings.

    +1 core tests.  The patch passed core unit tests.

    +1 contrib tests.  The patch passed contrib unit tests.

Test results: http://hudson.zones.apache.org/hudson/job/Mapreduce-Patch-h3.grid.sp2.yahoo.net/20/testReport/
Findbugs warnings: http://hudson.zones.apache.org/hudson/job/Mapreduce-Patch-h3.grid.sp2.yahoo.net/20/artifact/trunk/build/test/findbugs/newPatchFindbugsWarnings.html
Checkstyle results: http://hudson.zones.apache.org/hudson/job/Mapreduce-Patch-h3.grid.sp2.yahoo.net/20/artifact/trunk/build/test/checkstyle-errors.html
Console output: http://hudson.zones.apache.org/hudson/job/Mapreduce-Patch-h3.grid.sp2.yahoo.net/20/console

This message is automatically generated.

> More efficient SQL queries for DBInputFormat
> --------------------------------------------
>
>                 Key: MAPREDUCE-885
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-885
>             Project: Hadoop Map/Reduce
>          Issue Type: Improvement
>            Reporter: Aaron Kimball
>            Assignee: Aaron Kimball
>         Attachments: MAPREDUCE-885.2.patch, MAPREDUCE-885.3.patch, MAPREDUCE-885.4.patch, MAPREDUCE-885.5.patch, MAPREDUCE-885.6.patch, MAPREDUCE-885.patch
>
>
> DBInputFormat generates InputSplits by counting the available rows in a table, and selecting subsections of the table via the "LIMIT" and "OFFSET" SQL keywords. These are only meaningful in an ordered context, so the query also includes an "ORDER BY" clause on an index column. The resulting queries are often inefficient and require full table scans. Actually using multiple mappers with these queries can lead to O(n^2) behavior in the database, where n is the number of splits. Attempting to use parallelism with these queries is counter-productive.
> A better mechanism is to organize splits based on data values themselves, which can be performed in the WHERE clause, allowing for index range scans of tables, and can better exploit parallelism in the database.

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