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https://issues.apache.org/jira/browse/KAFKA-7497?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16743249#comment-16743249
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Matthias J. Sax commented on KAFKA-7497:
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You understanding seems correct. Also not the JavaDocs from `JoinWindows`
{quote}{{* In SQL-style you would express this join as}}
{{* }}
{{* SELECT * FROM stream1, stream2}}
{{* WHERE}}
{{* stream1.key = stream2.key}}
{{* AND}}
{{* stream1.ts - before <= stream2.ts AND stream2.ts <= stream1.ts + after}}
{quote}
I agree, it's not necessarily a public API change. However, it's might still be
a major change that we might want to back up with a KIP. Not sure. In the end,
it's an optimization to void two state stores, because one state store should
be sufficient to compute the self-join.
> Kafka Streams should support self-join on streams
> -------------------------------------------------
>
> Key: KAFKA-7497
> URL: https://issues.apache.org/jira/browse/KAFKA-7497
> Project: Kafka
> Issue Type: New Feature
> Components: streams
> Reporter: Robin Moffatt
> Priority: Major
> Labels: needs-kip
>
> There are valid reasons to want to join a stream to itself, but Kafka Streams
> does not currently support this ({{Invalid topology: Topic foo has already
> been registered by another source.}}). To perform the join requires creating
> a second stream as a clone of the first, and then doing a join between the
> two. This is a clunky workaround and results in unnecessary duplication of
> data.
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