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.