scala> (0 to 9).toDF("num").as[String].where('num === null).explain(extended = true)
== Parsed Logical Plan ==
'Filter ('num = null)
+- Project [value#137 AS num#139]
+- LocalRelation [value#137]
== Analyzed Logical Plan ==
num: int
Filter (num#139 = cast(null as int))
+- Project [value#137 AS num#139]
+- LocalRelation [value#137]
== Optimized Logical Plan ==
LocalRelation <empty>, [num#139]
== Physical Plan ==
LocalTableScan <empty>, [num#139]Nullability (NULL Value) Propagation
NullPropagation object is a operator optimization in Catalyst.
NullPropagation object is a Rule[LogicalPlan].