Global aggregation on unbounded file stream does not produce updates

[mbroecheler]

For example, when running the following script (on the loan data example):

IMPORT source.Applications;
AppCount := SELECT customer_id, SUM(amount) AS total_amount FROM Applications GROUP BY customer_id;

where the Applications source reads from local file continuously (i.e. "monitorIntervalMs" : "0" is NOT set), AppCount will be empty in the database. However, Applications are written to the database.

I assume this is a watermarking issue since the watermark does not advance, the aggregation never fires.

[mbroecheler]

Not sure if this is something we can "fix" as it is Flink behavior, but we should investigate if we can detect this case and warn about it or use a different watermarking strategy for this in our examples so people don't run into it.

[henneberger]

Let's add an warning when mixing bounded with unbounded streams if the file monitor config value isn't set.