Github user mn-mikke commented on a diff in the pull request:
https://github.com/apache/spark/pull/20858#discussion_r176847009
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/Expression.scala
---
@@ -699,3 +699,88 @@ abstract class TernaryExpression extends Expression {
* and Hive function wrappers.
*/
trait UserDefinedExpression
+
+/**
+ * The trait covers logic for performing null save evaluation and code
generation.
+ */
+trait NullSafeEvaluation extends Expression
+{
+ override def foldable: Boolean = children.forall(_.foldable)
+
+ override def nullable: Boolean = children.exists(_.nullable)
+
+ /**
+ * Default behavior of evaluation according to the default nullability
of NullSafeEvaluation.
+ * If a class utilizing NullSaveEvaluation override [[nullable]],
probably should also
+ * override this.
+ */
+ override def eval(input: InternalRow): Any =
+ {
+ val values = children.map(_.eval(input))
+ if (values.contains(null)) null
+ else nullSafeEval(values)
+ }
+
+ /**
+ * Called by default [[eval]] implementation. If a class utilizing
NullSaveEvaluation keep
+ * the default nullability, they can override this method to save
null-check code. If we need
+ * full control of evaluation process, we should override [[eval]].
+ */
+ protected def nullSafeEval(inputs: Seq[Any]): Any =
+ sys.error(s"The class utilizing NullSaveEvaluation must override
either eval or nullSafeEval")
+
+ /**
+ * Short hand for generating of null save evaluation code.
+ * If either of the sub-expressions is null, the result of this
computation
+ * is assumed to be null.
+ *
+ * @param f accepts a sequence of variable names and returns Java code
to compute the output.
+ */
+ protected def defineCodeGen(
+ ctx: CodegenContext,
+ ev: ExprCode,
+ f: Seq[String] => String): ExprCode = {
+ nullSafeCodeGen(ctx, ev, values => {
+ s"${ev.value} = ${f(values)};"
+ })
+ }
+
+ /**
+ * Called by expressions to generate null safe evaluation code.
+ * If either of the sub-expressions is null, the result of this
computation
+ * is assumed to be null.
+ *
+ * @param f a function that accepts a sequence of non-null evaluation
result names of children
+ * and returns Java code to compute the output.
+ */
+ protected def nullSafeCodeGen(
--- End diff --
@WeichenXu123 I do agree that there are strong similarities in the code.
If you take a look at `UniryExpression`, `BinaryExpression`,
`TernaryExpression`, you will see that methods responsible for null save
evaluation and code generation are the same except the number of parameters. My
intention has been to generalize the methods into the `NullSaveEvaluation`
trait and remove the original methods in a different PR once the trait is in. I
didn't want to create a big bang PR because of one additional function in API.
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