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From "Pradeep Kamath (JIRA)" <j...@apache.org>
Subject [jira] Created: (PIG-981) Merge join should restrict join key expressions to simple projects
Date Mon, 28 Sep 2009 21:35:15 GMT
Merge join should restrict join key expressions to simple projects

                 Key: PIG-981
                 URL: https://issues.apache.org/jira/browse/PIG-981
             Project: Pig
          Issue Type: Bug
    Affects Versions: 0.4.0
            Reporter: Pradeep Kamath
            Assignee: Pradeep Kamath

Currently merge join allows join key expressions to be arbitrary expressions with the assumption
that the expressions keep the sort order. Since currently only ascending sort order is supported,
the code checks at run times for sort order and catches the case where sort order is broken
because the join key expression is not order preserving. However there is a reason we should
restrict the join keys to projection of columns only:
 PIG-953 will enable pig to perform merge join  to work with loaders and store functions which
can internally index sorted data. These store functions can only create an index (and hence
lookup on the index) on raw data columns (and not expressions on the columns).
Hopefully this does not downgrade the usability of merge join much since if the expressions
can always be applied post join on the join columns and since the expressions are order preserving
they do not affect the outcome of the join. 

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