Github user wuchong commented on the issue:
https://github.com/apache/flink/pull/4355
The new created method `open(ctx)` and `close()` of `GeneratedAggregations`
are not called by `AggregateAggFunction` which is used in window aggregate. I
suggest to call the `open(ctx)` method but pass a `RuntimeContext` which throw
exceptions in every method to tell users `User Defined AggregateFunction is not
supported to call open() and close() in window`. But this can be addressed in
another issue.
BTW, I think the `AggregateCodeGenerator#generateAggregations` is too long
with 500+ LOC. I would like to refactor it if you have no objection @kaibozhou
@fhueske . I have created
[FLINK-7509](https://issues.apache.org/jira/browse/FLINK-7509) .
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