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https://issues.apache.org/jira/browse/FLINK-1267?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15098797#comment-15098797
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Stephan Ewen commented on FLINK-1267:
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I would close this as "won't fix". I cannot remember that anyone needed this,
as it is very easy to self implement inside a group reduce. The limit (group in
memory) is probably no real limit in practice.
> Add crossGroup operator
> -----------------------
>
> Key: FLINK-1267
> URL: https://issues.apache.org/jira/browse/FLINK-1267
> Project: Flink
> Issue Type: New Feature
> Components: DataSet API, Local Runtime, Optimizer
> Affects Versions: 0.7.0-incubating
> Reporter: Fabian Hueske
> Assignee: pietro pinoli
> Priority: Minor
>
> A common operator is to pair-wise compare or combine all elements of a group
> (there were two questions about this on the user mailing list, recently).
> Right now, this can be done in two ways:
> 1. {{groupReduce}}: consume and store the complete iterator in memory and
> build all pairs
> 2. do a self-{{Join}}: the engine builds all pairs of the full symmetric
> Cartesian product.
> Both approaches have drawbacks. The {{groupReduce}} variant requires that the
> full group fits into memory and is more cumbersome to implement for the user,
> but pairs can be arbitrarily built. The self-{{Join}} approach pushes most of
> the work into the system, but the execution strategy does not treat the
> self-join different from a regular join (both identical inputs are shuffled,
> etc.) and always builds the full symmetric Cartesian product.
> I propose to add a dedicated {{crossGroup()}} operator, that offers this
> functionality in a proper way.
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