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https://issues.apache.org/jira/browse/FLINK-7029?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16066456#comment-16066456
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Aljoscha Krettek commented on FLINK-7029:
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I actually don't like the variants without an explicit {{KeySelector}} because
it's not possible to statically type check these cases (either tuple keys or
POJO field keys). Using a {{KeySelector}} makes the key type explicit and also
makes it explicit in the {{WindowFunction}}.
As you noticed, the key type is always {{Tuple}} when not using a
{{KeySelector}} and I don't see a way of making that type more explicit when
only selecting one field.
> Documentation for WindowFunction is confusing
> ---------------------------------------------
>
> Key: FLINK-7029
> URL: https://issues.apache.org/jira/browse/FLINK-7029
> Project: Flink
> Issue Type: Improvement
> Components: Documentation
> Reporter: Felix Neutatz
> Priority: Trivial
> Labels: documentation
>
> Hi,
> in the [example of the WindowFunction in the
> documentation|https://ci.apache.org/projects/flink/flink-docs-release-1.3/dev/windows.html#windowfunction---the-generic-case]
> we use WindowFunction<Tuple<String, Long>, String, String, TimeWindow>. That
> means that our key data-type is a String. For me, this is highly confusing,
> since we can only have a String data type for the key, if we implement a
> custom key selector. Usually people, especially beginners, will use something
> like keyBy(ID), keyBy("attributeName"), which will always return a tuple e.g.
> a Tuple1<String>. It would be great if somebody could change this to a tuple
> key type in the example. I am sure this might help beginners to understand
> that by default the key type is a tuple.
> Moreover, another suggestion would be that we overwrite keyBy() in a way that
> if we just get one attribute, we return this type directly instead of
> wrapping it in a Tuple1.
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