[ https://issues.apache.org/jira/browse/MAPREDUCE-885?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]
Aaron Kimball updated MAPREDUCE-885: ------------------------------------ Status: Patch Available (was: Open) > 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. -- This message is automatically generated by JIRA. - You can reply to this email to add a comment to the issue online.