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https://issues.apache.org/jira/browse/FLINK-2991?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15141672#comment-15141672
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ASF GitHub Bot commented on FLINK-2991:
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Github user aljoscha commented on the pull request:
https://github.com/apache/flink/pull/1605#issuecomment-182583799
You can also do just a "fold". The window function is for when you need the
meta information about the window because the fold function does not get that.
> Extend Window Operators to Allow Efficient Fold Operation
> ---------------------------------------------------------
>
> Key: FLINK-2991
> URL: https://issues.apache.org/jira/browse/FLINK-2991
> Project: Flink
> Issue Type: Improvement
> Components: Streaming
> Reporter: Aljoscha Krettek
> Assignee: Aljoscha Krettek
>
> Right now, a window fold is implemented as a WindowFunction that gets all the
> elements as input. No pre-aggregation is performed. The window operator
> should be extended to also allow the fold to also be pre-aggregated.
> This requires changing the signature of the {{WindowBuffer}} so that it can
> emit a type other than the input type.
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