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Hadoop QA commented on MAPREDUCE-885: ------------------------------------- -1 overall. Here are the results of testing the latest attachment http://issues.apache.org/jira/secure/attachment/12416936/MAPREDUCE-885.patch against trunk revision 805324. +1 @author. The patch does not contain any @author tags. +1 tests included. The patch appears to include 4 new or modified tests. -1 patch. The patch command could not apply the patch. Console output: http://hudson.zones.apache.org/hudson/job/Mapreduce-Patch-vesta.apache.org/490/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.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. -- This message is automatically generated by JIRA. - You can reply to this email to add a comment to the issue online.