StreamingRelation is the LogicalPlan of the DataFrame being the result of executing DataFrameReader.stream method.
val reader = spark.read
val helloDF = reader.stream("hello")
scala> helloDF.explain(true)
== Parsed Logical Plan ==
FileSource[hello]
== Analyzed Logical Plan ==
id: bigint
FileSource[hello]
== Optimized Logical Plan ==
FileSource[hello]
== Physical Plan ==
java.lang.AssertionError: assertion failed: No plan for FileSource[hello]