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https://issues.apache.org/jira/browse/FLINK-4578?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15463363#comment-15463363
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Gabor Gevay commented on FLINK-4578:
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A hacky solution might be to create some dummy {{Keys}} object from the
aggregate field, and call {{computeLogicalKeyPositions}} on it, so that we know
the flat position of the key field.
> AggregateOperator incorrectly sets ForwardedField with nested composite types
> -----------------------------------------------------------------------------
>
> Key: FLINK-4578
> URL: https://issues.apache.org/jira/browse/FLINK-4578
> Project: Flink
> Issue Type: Bug
> Components: DataSet API
> Reporter: Gabor Gevay
>
> When an aggregation is called on a grouped DataSet,
> {{AggregateOperator.translateToDataFlow}} tries to determine whether the
> field that is being aggregated is the same field that the grouping is based
> on. If this is not the case, then it adds the ForwardedField property for the
> key field.
> However, the mechanism that makes this decision breaks when there are nested
> composite types involved, because it gets the key positions with
> {{getKeys().computeLogicalKeyPositions()}}, which returns the _flat_
> positions, whereas the position of the field to aggregate is counted only on
> the outer type.
> Example code: https://github.com/ggevay/flink/tree/agg-bad-forwarded-fields
> Here, I have changed the WordCount example to have the type
> {{Tuple3<Tuple2<Byte,Byte>, String, Integer>}}, and do {{.groupBy(1).sum(2)}}
> (which groups by the String field and sums the Integer field). If you set a
> breakpoint into {{AggregateOperator.translateToDataFlow}}, you can see that
> {{logicalKeyPositions}} contains 2, and {{fields}} also contains 2, which
> causes {{keyFieldUsedInAgg}} to be erroneously set to true. The problem is
> caused by the Tuple2 being counted as 2 fields in {{logicalKeyPositions}},
> but only 1 field in {{fields}}.
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