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https://issues.apache.org/jira/browse/FLINK-703?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14384529#comment-14384529
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Fabian Hueske commented on FLINK-703:
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Hi, that's a very good example and shows that the description of this issue is
incomplete ;-)
Key expressions (such as {{"*"}} and {{"_"}}) are supported for Pojo, Tuple,
and CaseClass types.
If you would change your example in a way, that you join against DataSet of
primitive types (such as DataSet<Integer>) it would no longer work. Since
primitive types are not composite, the only possible key expression is the
wildcard which selects the full type. We handle this case in several places of
the API as special case. See for example in
[DataSink.java|https://github.com/apache/flink/blob/master/flink-java/src/main/java/org/apache/flink/api/java/operators/DataSink.java]
in function {{sortLocalOutput(String fieldExpression, Order order)}} (around
line 180).
So this issue would be mean to add similiar functionality to the key definition
functions of join, coGroup, and grouping. For that, we need to check if the
type is a valid key type ({{TypeInformation.isKeyType()}}) and if the type is
an atomic type, set the key to int[]\{0\}
> Use complete element as join key.
> ---------------------------------
>
> Key: FLINK-703
> URL: https://issues.apache.org/jira/browse/FLINK-703
> Project: Flink
> Issue Type: Improvement
> Reporter: GitHub Import
> Assignee: Chiwan Park
> Priority: Trivial
> Labels: github-import
> Fix For: pre-apache
>
>
> In some situations such as semi-joins it could make sense to use a complete
> element as join key.
> Currently this can be done using a key-selector function, but we could offer
> a shortcut for that.
> This is not an urgent issue, but might be helpful.
> ---------------- Imported from GitHub ----------------
> Url: https://github.com/stratosphere/stratosphere/issues/703
> Created by: [fhueske|https://github.com/fhueske]
> Labels: enhancement, java api, user satisfaction,
> Milestone: Release 0.6 (unplanned)
> Created at: Thu Apr 17 23:40:00 CEST 2014
> State: open
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