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https://issues.apache.org/jira/browse/PIG-1916?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13053407#comment-13053407
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Zhijie Shen commented on PIG-1916:
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As Daniel suggested, I differ the process of visiting LOCross: for the
top-level LOCross, I keep the original logic; for the nested LOCross, I
generate a POCross instead and place it into the correct place of the physical
plan. After that, I've found the physical and map/reduce plan is generated
correctly. However, the pipeline was still broken because the getNext()
function of POCross hasn't been implemented. I recall that you've mentioned
there's POCross in previous version of Pig. I found out the code and reuse the
getNext() function. I found it almost worked! There were some bugs, which were
fixed by me. Finally, the nested cross basically works! Please refer to the
newest patch for the code update.
> Nested cross
> ------------
>
> Key: PIG-1916
> URL: https://issues.apache.org/jira/browse/PIG-1916
> Project: Pig
> Issue Type: New Feature
> Components: impl
> Reporter: Daniel Dai
> Labels: gsoc2011
> Fix For: 0.10
>
> Attachments: PIG-1916_1.patch, PIG-1916_2.patch
>
>
> It is useful to have cross inside foreach nested statement. One typical use
> case for nested foreach is after cogroup two relations, we want to flatten
> the records of the same key, and do some processing. This is naturally to be
> achieved by cross. Eg:
> {code}
> C = cogroup user by uid, session by uid;
> D = foreach C {
> crossed = cross user, session; -- To flatten two input bags
> filtered = filter crossed by user::region == session::region;
> result = foreach crossed generate processSession(user::age, user::gender,
> session::ip); --Nested foreach Jira: PIG-1631
> generate result;
> }
> {code}
> If we don't have cross, user have to write a UDF process the bag user,
> session. It is much harder than a UDF process flattened tuples. This is
> especially true when we have nested foreach statement(PIG-1631).
> This is a candidate project for Google summer of code 2011. More information
> about the program can be found at http://wiki.apache.org/pig/GSoc2011
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