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https://issues.apache.org/jira/browse/KAFKA-14748?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17694652#comment-17694652
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Guozhang Wang commented on KAFKA-14748:
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I agree for Stream-Stream joins now, since for the case "no right hand side
value found" we would not emit immediately so it is the case with
"key-extractor returns null". But for table-table FK-joins, today the former
case would emit while the latter case would not?
> Relax non-null FK left-join requirement
> ---------------------------------------
>
> Key: KAFKA-14748
> URL: https://issues.apache.org/jira/browse/KAFKA-14748
> Project: Kafka
> Issue Type: Improvement
> Components: streams
> Reporter: Matthias J. Sax
> Priority: Major
>
> Kafka Streams enforces a strict non-null-key policy in the DSL across all
> key-dependent operations (like aggregations and joins).
> This also applies to FK-joins, in particular to the ForeignKeyExtractor. If
> it returns `null`, it's treated as invalid. For left-joins, it might make
> sense to still accept a `null`, and add the left-hand record with an empty
> right-hand-side to the result.
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