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https://issues.apache.org/jira/browse/HADOOP-2921?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12575093#action_12575093
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Doug Cutting commented on HADOOP-2921:
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> where the value encodes a row. The sort field is embedded inside the row
What I'm suggesting is that, rather than a null key, you use a null value and
put the row in the key. Why doesn't that work for you? Then you can use
existing key-oriented tools for sorting.
> align map splits on sorted files with key boundaries
> ----------------------------------------------------
>
> Key: HADOOP-2921
> URL: https://issues.apache.org/jira/browse/HADOOP-2921
> Project: Hadoop Core
> Issue Type: New Feature
> Affects Versions: 0.16.0
> Reporter: Joydeep Sen Sarma
>
> (this is something that we have implemented in the application layer - may be
> useful to have in hadoop itself).
> long term log storage systems often keep data sorted (by some sort-key).
> future computations on such files can often benefit from this sort order. if
> the job requires grouping by the sort-key - then it should be possible to do
> reduction in the map stage itself.
> this is not natively supported by hadoop (except in the degenerate case of 1
> map file per task) since splits can span the sort-key. however aligning the
> data read by the map task to sort key boundaries is straightforward - and
> this would be a useful capability to have in hadoop.
> the definition of the sort key should be left up to the application (it's not
> necessarily the key field in a Sequencefile) through a generic interface -
> but otherwise - the sequencefile and text file readers can use the extracted
> sort key to align map task data with key boundaries.
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