More efficient SQL queries for DBInputFormat
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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.
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