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https://issues.apache.org/jira/browse/FLINK-3477?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15246167#comment-15246167
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ASF GitHub Bot commented on FLINK-3477:
---------------------------------------
Github user greghogan commented on a diff in the pull request:
https://github.com/apache/flink/pull/1517#discussion_r60104097
--- Diff:
flink-core/src/main/java/org/apache/flink/api/common/operators/base/ReduceOperatorBase.java
---
@@ -54,6 +54,32 @@
@Internal
public class ReduceOperatorBase<T, FT extends ReduceFunction<T>> extends
SingleInputOperator<T, T, FT> {
+ /**
+ * An enumeration of hints, optionally usable to tell the system
exactly how to execute the combiner phase
+ * of a reduce.
+ * (Note: The final reduce phase (after combining) is currently always
executed by a sort-based strategy.)
+ */
+ public enum CombineHint {
--- End diff --
Should we consider an additional flag {{CombineHint.NONE}} for cases where
the expected number of duplicate keys is relatively small (i.e. FLINK-3279)?
> Add hash-based combine strategy for ReduceFunction
> --------------------------------------------------
>
> Key: FLINK-3477
> URL: https://issues.apache.org/jira/browse/FLINK-3477
> Project: Flink
> Issue Type: Sub-task
> Components: Local Runtime
> Reporter: Fabian Hueske
> Assignee: Gabor Gevay
>
> This issue is about adding a hash-based combine strategy for ReduceFunctions.
> The interface of the {{reduce()}} method is as follows:
> {code}
> public T reduce(T v1, T v2)
> {code}
> Input type and output type are identical and the function returns only a
> single value. A Reduce function is incrementally applied to compute a final
> aggregated value. This allows to hold the preaggregated value in a hash-table
> and update it with each function call.
> The hash-based strategy requires special implementation of an in-memory hash
> table. The hash table should support in place updates of elements (if the
> updated value has the same size as the new value) but also appending updates
> with invalidation of the old value (if the binary length of the new value
> differs). The hash table needs to be able to evict and emit all elements if
> it runs out-of-memory.
> We should also add {{HASH}} and {{SORT}} compiler hints to
> {{DataSet.reduce()}} and {{Grouping.reduce()}} to allow users to pick the
> execution strategy.
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