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https://issues.apache.org/jira/browse/FLINK-22737?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17354929#comment-17354929
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Jark Wu commented on FLINK-22737:
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It seems no option can make every one happy.
I would be fine with NULL if we can clearly state on docs the default value of
CURRENT_WATERMARK before any watermark emitted is **{{NULL}}**, and users
should be careful when using it to drop late data, should use {{WHERE
CURRENT_WATERMARK() IS NULL OR rowtime > CURRENT_WATERMARK()}}.
> 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
> Attachments: screenshot-2021-05-31_14-22-43.png
>
>
> 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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