Github user marmbrus commented on the issue:
https://github.com/apache/spark/pull/17268
Sorry I'm still not sure if this is a good idea.
Why disallow the following,
```scala
spark
.readStream
.withWatermark("eventTime", "10 seconds")
.dropDuplicates("id")
```
But not this query?
```scala
spark
.readStream
.withWatermark("eventTime", "10 seconds")
.groupBy("id")
.agg(first(struct($"*")))
```
I think its an incredibly difficult problem to tell people if their query
is scalable without seeing the data. I'm not sure if we want to do it
piecemeal as I'm concerned that is more confusing.
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