[SPARK-32669][SQL][TEST] Expression unit tests should explore all cases that can lead to null result

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/ComplexTypeSuite.scala

@@ -143,21 +143,26 @@ class ComplexTypeSuite extends SparkFunSuite with ExpressionEvalHelper {
   }
 
   test("GetArrayStructFields") {
-    val typeAS = ArrayType(StructType(StructField("a", IntegerType, false) :: Nil))
-    val typeNullAS = ArrayType(StructType(StructField("a", IntegerType) :: Nil))
-    val arrayStruct = Literal.create(Seq(create_row(1)), typeAS)
-    val nullArrayStruct = Literal.create(null, typeNullAS)
-
-    def getArrayStructFields(expr: Expression, fieldName: String): GetArrayStructFields = {
-      expr.dataType match {
-        case ArrayType(StructType(fields), containsNull) =>
-          val field = fields.find(_.name == fieldName).get
-          GetArrayStructFields(expr, field, fields.indexOf(field), fields.length, containsNull)
-      }
+    // test 4 types: struct field nullability X array element nullability
+    val type1 = ArrayType(StructType(StructField("a", IntegerType) :: Nil))
+    val type2 = ArrayType(StructType(StructField("a", IntegerType, nullable = false) :: Nil))
+    val type3 = ArrayType(StructType(StructField("a", IntegerType) :: Nil), containsNull = false)
+    val type4 = ArrayType(
+      StructType(StructField("a", IntegerType, nullable = false) :: Nil), containsNull = false)
+
+    val input1 = Literal.create(Seq(create_row(1)), type4)
+    val input2 = Literal.create(Seq(create_row(null)), type3)
+    val input3 = Literal.create(Seq(null), type2)
+    val input4 = Literal.create(null, type1)
+
+    def getArrayStructFields(expr: Expression, fieldName: String): Expression = {
+      ExtractValue.apply(expr, Literal.create(fieldName, StringType), _ == _)
     }
 
-    checkEvaluation(getArrayStructFields(arrayStruct, "a"), Seq(1))
-    checkEvaluation(getArrayStructFields(nullArrayStruct, "a"), null)
+    checkEvaluation(getArrayStructFields(input1, "a"), Seq(1))
+    checkEvaluation(getArrayStructFields(input2, "a"), Seq(null))
+    checkEvaluation(getArrayStructFields(input3, "a"), Seq(null))
+    checkEvaluation(getArrayStructFields(input4, "a"), null)
   }
 
   test("SPARK-32167: nullability of GetArrayStructFields") {

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/ExpressionEvalHelper.scala

@@ -40,6 +40,11 @@ import org.apache.spark.util.Utils
 
 /**
  * A few helper functions for expression evaluation testing. Mixin this trait to use them.
+ *
+ * Note: when you write unit test for an expression and call `checkEvaluation` to check the result,
+ *       please make sure that you explore all the cases that can lead to null result (including
+ *       null in struct fields, array elements and map values). The framework will test the
+ *       nullability flag of the expression automatically.
  */
 trait ExpressionEvalHelper extends ScalaCheckDrivenPropertyChecks with PlanTestBase {
   self: SparkFunSuite =>