Github user fhueske commented on the issue:
https://github.com/apache/flink/pull/6253
It's a good question whether to add a new class or not. Right now,
extending the implementation of `ExistingField` seems like a better approach to
me. Once we add a `ParsingExistingField` extractor that can be configured with
a timestamp format, the ISO date String support in `ExistingField` would be
obsolete, but IMO that's not a big problem.
+1 to change the implementation to extend `ExistingField`.
Regarding the tests, I think `ExistingField` is used in a few ITCases, but
there are no unit tests yet.
Adding unit tests is a bit tricky, because we would need to integrate it
with the code generator, etc.
So, a big +1 if you would like to look into that, but I'd also be fine by
adding another ITCase.
For the documentation, we might want to extend the bullet point about
`timestampExtractor` in the [Defining a Rowtime
Attribute](https://ci.apache.org/projects/flink/flink-docs-release-1.5/dev/table/sourceSinks.html#defining-a-rowtime-attribute)
section.
Thanks, Fabian
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