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https://issues.apache.org/jira/browse/MAPREDUCE-885?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12747277#action_12747277
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Enis Soztutar commented on MAPREDUCE-885:
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Data driven splits are really neat. Just a few suggestions
- We can add a getSplitter(int sqlDataType) method to DDDBIF and move sql type
-> DBSplitter instance mapping, so that classes extending it can easily
override this logic, for skewed data, etc.
- Introduce DDDBRR extending DBRR in DDDBIF and move getDataBasedSelectQuery()
as an overridden implementation of getSelectQuery().
- Do we need mapred.lib.db.DDDBIF since it is introduced as deprecated. I know
that lot's of legacy code is using the old API, but adding a already deprecated
class seems odd.
> 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.
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