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https://issues.apache.org/jira/browse/FLINK-3479?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Aljoscha Krettek closed FLINK-3479.
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Resolution: Won't Do
I'm closing this for now because the DataSet API is more in maintenance mode
and we don't actively work on new features there.
> Add hash-based strategy for CombineFunction
> -------------------------------------------
>
> Key: FLINK-3479
> URL: https://issues.apache.org/jira/browse/FLINK-3479
> Project: Flink
> Issue Type: Sub-task
> Components: Runtime / Task
> Reporter: Fabian Hueske
> Priority: Minor
>
> This issue is similar to FLINK-3477 but adds a hash-based strategy for
> {{CombineFunction}} instead of {{ReduceFunction}}.
> The interface of {{CombineFunction}} differs from {{ReduceFunction}} by
> providing an {{Iterable<T>}} instead of two {{T}} values. Hence, if the
> {{Iterable<T>}} provides two values, we can do the same as with a
> {{ReduceFunction}}.
> At the moment, {{CombineFunction}} is wrapped in a {{GroupCombineFunction}}
> and hence executed using the {{GroupReduceCombineDriver}}.
> We should add dedicated two dedicated drivers: {{CombineDriver}} and
> {{ChainedCombineDriver}} and two driver strategies: {{HASH_COMBINE}} and
> {{SORT_COMBINE}}.
> If FLINK-3477 is resolved, we can reuse the hash-table.
> We should also add compiler hints to `DataSet.reduceGroup()` and
> `Grouping.reduceGroup()` to allow users to select between a {{SORT}} and
> {{HASH}} based combine strategies ({{HASH}} will only be applicable to
> {{CombineFunction}} and not {{GroupCombineFunction}}).
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