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https://issues.apache.org/jira/browse/MAPREDUCE-885?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12755554#action_12755554
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Hudson commented on MAPREDUCE-885:
----------------------------------
Integrated in Hadoop-Mapreduce-trunk #83 (See
[http://hudson.zones.apache.org/hudson/job/Hadoop-Mapreduce-trunk/83/])
. More efficient SQL queries for DBInputFormat. Contributed by Aaron
Kimball.
> 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
> Fix For: 0.21.0
>
> 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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