dongjoon-hyun commented on a change in pull request #23867: [SPARK-26965][SQL] 
Makes ElementAt nullability more precise
URL: https://github.com/apache/spark/pull/23867#discussion_r261003131
 
 

 ##########
 File path: 
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/complexTypeExtractors.scala
 ##########
 @@ -281,10 +266,53 @@ case class GetArrayItem(child: Expression, ordinal: 
Expression)
 }
 
 /**
- * Common base class for [[GetMapValue]] and [[ElementAt]].
+ * Common trait for [[GetArrayItem]] and [[ElementAt]].
+ */
+trait GetArrayItemUtil {
+
+  /** `Null` is returned for invalid ordinals. */
+  protected def computeNullabilityFromArray(child: Expression, ordinal: 
Expression): Boolean = {
+    if (ordinal.foldable && !ordinal.nullable) {
+      val intOrdinal = ordinal.eval().asInstanceOf[Number].intValue()
+      child match {
+        case CreateArray(ar) if intOrdinal < ar.length =>
+          ar(intOrdinal).nullable
+        case GetArrayStructFields(CreateArray(elements), field, _, _, _)
+          if intOrdinal < elements.length =>
+          elements(intOrdinal).nullable || field.nullable
+        case _ =>
+          true
+      }
+    } else {
+      true
+    }
+  }
+}
+
+/**
+ * Common trait for [[GetMapValue]] and [[ElementAt]].
  */
+trait GetMapValueUtil extends BinaryExpression with ImplicitCastInputTypes {
+
+  private val child = left
+  private val key = right
+
+  /** `Null` is returned for invalid ordinals. */
+  protected def computeNullabilityFromMap: Boolean = if (key.foldable && 
!key.nullable) {
+    val keyObj = key.eval()
+    child match {
 
 Review comment:
   If you want to keep the clear meaning, you may declare them inside of this 
function. But, since we are not using `parameters`, I think we already have 
some assumptions that this is inside `BinaryExpression`.
   ```scala
   protected def computeNullabilityFromMap: Boolean = {
       val child = left
       val key = right
       ....
   }
   ```

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