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https://issues.apache.org/jira/browse/FLINK-22737?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17353304#comment-17353304
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Jark Wu commented on FLINK-22737:
---------------------------------

Hi [~airblader], [~twalthr], I prefer to return TIMESTAMP/LTZ rather than 
BIGINT, because as said in the JIRA description, it is mainly used to compare 
with event time to determine late events. So a same type with event time 
attribute would make the comparison easier and less error-prone (no 
conversion).  

What about asking {{CURRENT_WATERMARK}} to accept an event-time attribute 
argument? I believe the event-time column should exist if users want to access 
watermark. This can make event time column and CURRENT_WATERMARK return same 
type,  also avoid returning NULL values. 

> 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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