Expected behavior for DataFrame.unionAll

[Justin Yip]

Hello,

I am experimenting with DataFrame. I tried to construct two DataFrames with:
1. case class A(a: Int, b: String)
scala> adf.printSchema()
root
 |-- a: integer (nullable = false)
 |-- b: string (nullable = true)

2. case class B(a: String, c: Int)
scala> bdf.printSchema()
root
 |-- a: string (nullable = true)
 |-- c: integer (nullable = false)


Then I unioned the these two DataFrame with the unionAll function, and I
get the following schema. It is kind of a mixture of A and B.

scala> val udf = adf.unionAll(bdf)
scala> udf.printSchema()
root
 |-- a: string (nullable = false)
 |-- b: string (nullable = true)

The unionAll documentation says it behaves like the SQL UNION ALL function.
However, unioning incompatible types is not well defined for SQL. Is there
any expected behavior for unioning incompatible data frames?

Thanks.

Justin