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https://issues.apache.org/jira/browse/FLINK-22737?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17354250#comment-17354250
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Ingo Bürk edited comment on FLINK-22737 at 5/31/21, 7:14 AM:
-------------------------------------------------------------

I discussed this again with Timo, and we will go with the Unix epoch instead of 
NULL now. To summarize:
 * CURRENT_WATERMARK(<column>)
 * Error if the provided column is not a rowtime column
 * The returned type is that of the provided column (TIMESTAMP / 
TIMESTAMP_LTZ), but we force the precision to 3.
 * If no watermark has been emitted yet, we return the Unix epoch.

Edit: Sorry, I may have misunderstood my discussion with Timo. 


was (Author: airblader):
I discussed this again with Timo, and we will go with the Unix epoch instead of 
NULL now. To summarize:
 * CURRENT_WATERMARK(<column>)
 * Error if the provided column is not a rowtime column
 * The returned type is that of the provided column (TIMESTAMP / 
TIMESTAMP_LTZ), but we force the precision to 3.
 * If no watermark has been emitted yet, we return the Unix epoch.

> 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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