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https://issues.apache.org/jira/browse/FLINK-22737?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17353138#comment-17353138
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Ingo Bürk commented on FLINK-22737:
-----------------------------------
I decided with [~twalthr] that we'll make this function return TIMESTAMP_LTZ to
be in line with other functions, and return NULL if either no event-time
attribute exists or no watermark has yet been emitted. It'd actually be very
difficult to determine whether an event-time attribute exists, so we rather map
the value returned by TimerService#currentWatermark() (which is MIN_VALUE in
both cases) correspondingly, as NULL requires careful handling, but
semantically fits better than a "placeholder" date like 1/1/1970.
> 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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