Github user cloud-fan commented on the issue:
https://github.com/apache/spark/pull/15979
```
val x: Dataset[String, Option[(String, String)]] = ...
x.groupByKey(_._1).mapValues(_._2).agg(someAgg)
```
Does it work before?
Please see the discussion in the JIRA:
https://issues.apache.org/jira/browse/SPARK-18251
Ideally we have a map between type T and catalyst schema, and `Option[T]`
maps to the same catalyst schema with `T`, with additional null handling. We
shouldn't change this mapping, which means we can't use a single field struct
type to represent `Option[T]`.
It's still possible to support `Option[T]` completely(without breaking
backward compatibility), but that may need a lof of hacky code and special
handling, I don't think it worth, as we can easy work around it, by `Tuple1`.
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