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https://issues.apache.org/jira/browse/FLINK-2144?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15294791#comment-15294791
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Aljoscha Krettek commented on FLINK-2144:
-----------------------------------------

We could provide pre-made FoldFunctions. What would the types of these be, i.e. 
what's the input and the output/accumulator of the FoldFunction?

> Incremental count, average, and variance for windows
> ----------------------------------------------------
>
>                 Key: FLINK-2144
>                 URL: https://issues.apache.org/jira/browse/FLINK-2144
>             Project: Flink
>          Issue Type: New Feature
>          Components: Streaming
>            Reporter: Gabor Gevay
>            Priority: Minor
>              Labels: statistics
>
> By count I mean the number of elements in the window.
> These can be implemented very efficiently building on FLINK-2143:
> Store: O(1)
> Evict: O(1)
> emitWindow: O(1)



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