[FLINK-6011] Support TUMBLE, HOP, SESSION window in streaming SQL.

[haohui]

This PR adds supports for the TUMBLE, HOP, and SESSION windows in Flink.

The work of supporting WindowStart and WindowEnd expressions will be deferred to FLINK-6012.

[fhueske]

Thanks for the PR @haohui!
+1 to merge.
I've been waiting for this feature :-)

[shaoxuan-wang]

So we finally got those supported by Calcite 1.12？Really excited to see those features supported in flinkSQL. Thanks @haohui.

[twalthr]

Thanks @haohui. Great that we support windows in SQL soon. I had some minor comments. Could you also add some documentation in the SQL section (maybe by copying paragraphs from the Table API section)?

[twalthr]

Do we already support the START/END functions? We should let them unsupported until they are implemented and tested.

[twalthr]

Does Calcite ensure that operands can only be literals, no input reference?

[haohui]

I just tried out Calcite did not stop you from passing something like a {{RexCall}}. So yes, it can be dynamic.

One question: whether Flink actually supports GroupWindow that has a dynamic size? Maybe I'm wrong but it does not seem so.

If the answer is no maybe we should check that whether it is a RexLiteral?

[fhueske]

Flink does only support windows with fixed configuration (SESSION windows have variable length, but the gap parameter is fixed). I'm also not sure if that would make sense. It's quite hard to reason about the behavior of a window with variable parameters, IMO.

[haohui]

Agree. 97d1a45 will throw an TableException if the configuration is not fixed.

[fhueske]

Thanks for the update @haohui!
Will merge this PR.