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https://issues.apache.org/jira/browse/FLINK-8144?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16267762#comment-16267762
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ASF GitHub Bot commented on FLINK-8144:
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Github user fhueske commented on the issue:
https://github.com/apache/flink/pull/5063
A punctuated watermark assigner can emit `null` instead of a watermark. So
using a punctuated watermark assigner does not mean that for each event a
watermark must be emitted. The motivation for punctuated watermarks is to react
to specific events that carry watermark information and not to all events (see
[JavaDocs](https://github.com/apache/flink/blob/master/flink-streaming-java/src/main/java/org/apache/flink/streaming/api/functions/AssignerWithPunctuatedWatermarks.java)).
Of course there's nothing that prevents users from emitting one watermark per
event but as I said this was not the intended use case.
A `lastProcessedWatermark` state variable would improve the situation a bit
by acting as an early out for the `onTimer()` method. However, the system would
still need to checkpoint and handle significantly more timers than with the
current implementation.
> Optimize the timer logic in RowTimeUnboundedOver
> ------------------------------------------------
>
> Key: FLINK-8144
> URL: https://issues.apache.org/jira/browse/FLINK-8144
> Project: Flink
> Issue Type: Bug
> Components: Table API & SQL
> Reporter: Dian Fu
> Assignee: Dian Fu
> Fix For: 1.5.0
>
>
> Currently the logic of {{RowTimeUnboundedOver}} is as follows:
> 1) When element comes, buffer it in MapState and and register a timer at
> {{current watermark + 1}}
> 2) When event timer triggered, scan the MapState and find the elements below
> the current watermark and process it. If there are remaining elements to
> process, register a new timer at {{current watermark + 1}}.
> Let's assume that watermark comes about 5 seconds later than the event on
> average, then we will scan about 5000 times the MapState before actually
> processing the events.
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