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From "Yan Zhou (JIRA)" <j...@apache.org>
Subject [jira] Created: (PIG-1306) [zebra] Support of locally sorted input splits
Date Thu, 18 Mar 2010 19:58:27 GMT
[zebra] Support of locally sorted input splits

                 Key: PIG-1306
                 URL: https://issues.apache.org/jira/browse/PIG-1306
             Project: Pig
          Issue Type: Improvement
            Reporter: Yan Zhou

Current Zebra supports sorted or unsorted input splits on sorted table or sorted table unions.
The sorted input splits are based upon key ranges which do not overlap. And the splits are
basically globally sorted in that they are locally sorted, and their key ranges do not overlap.

The biggest problem of the key-range splits are performance hits suffered if data skew is
present, particularly if a key range contains a duplicate key solely which makes the data
trunk of the duplicate keys virtually unsplittable regardless how many mappers are available:
it just has to be processed by a single mapper.

On the other hand, there are scenarios when the globally sorted splits are a over-kill and
only locally sorted splits are good enough. Examples are the use of Zebra sorted tables as
the probe table in a map-side merge inner join.

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