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https://issues.apache.org/jira/browse/FLINK-22737?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17353427#comment-17353427
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Jark Wu commented on FLINK-22737:
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I have some concerns when returning NULL, because it involves
three-valued-logic when comparing which may confuse users at sometimes. I think
returning 0 as initial watermark is fine which represents "1970-01-01 00:00:00"
if is TIMESTAMP type, or "1970-01-01 00:00:00Z" if is TIMESTAMP_LTZ type.
> 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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