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https://issues.apache.org/jira/browse/FLINK-18830?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17203111#comment-17203111
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Aljoscha Krettek commented on FLINK-18830:
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If it's for the Table API then we should keep it as an internal implementation.
I don't think we should extend the DataStream API to support more (window) join
types.
If users need joins they should use the Table API, so we shouldn't recommend to
users to use the DataStream API for that. Which means that we shouldn't add
such operations to the DataStream API.
> JoinCoGroupFunction and FlatJoinCoGroupFunction work incorrectly for outer
> join when one side of coGroup is empty
> -----------------------------------------------------------------------------------------------------------------
>
> Key: FLINK-18830
> URL: https://issues.apache.org/jira/browse/FLINK-18830
> Project: Flink
> Issue Type: Bug
> Components: API / DataStream
> Affects Versions: 1.11.1
> Reporter: liupengcheng
> Priority: Major
>
> Currently, The {{JoinCoGroupFunction}} and {{FlatJoinCoGroupFunction}} in
> JoinedStreams doesn't respect the join type, it's been implemented as doing
> join within a two-level loop. However, this is incorrect for outer join when
> one side of the coGroup is empty.
> {code}
> public void coGroup(Iterable<T1> first, Iterable<T2> second,
> Collector<T> out) throws Exception {
> for (T1 val1: first) {
> for (T2 val2: second) {
> wrappedFunction.join(val1, val2, out);
> }
> }
> }
> {code}
> The above code is the current implementation, suppose the first input is
> non-empty, and the second input is an empty iterator, then the join
> function(`wrappedFunction`) will never be called. This will cause no data to
> be emitted for a left outer join.
> So I propose to consider join type here, and handle this case, e.g., for left
> outer join, we can emit record with right side set to null here if the right
> side is empty or can not find any match in the right side.
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