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https://issues.apache.org/jira/browse/FLINK-22737?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17353405#comment-17353405
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David Anderson commented on FLINK-22737:
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Semantically, I would argue that returning 0 as an initial watermark is
perfectly sensible. Since no timestamps can be less than that watermark,
nothing will be considered late, which is correct. 0 is a perfectly logical
return value, and shouldn't require any special handling.
> Add support for CURRENT_WATERMARK to SQL
> ----------------------------------------
>
> Key: FLINK-22737
> URL: https://issues.apache.org/jira/browse/FLINK-22737
> Project: Flink
> Issue Type: Sub-task
> Components: Table SQL / API
> Reporter: David Anderson
> Assignee: Ingo Bürk
> Priority: Major
>
> With a built-in function returning the current watermark, one could operate
> on late events without resorting to using the DataStream API.
> Called with zero parameters, this function returns the current watermark for
> the current row – if there is an event time attribute. Otherwise, it returns
> NULL.
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