[SPARK-26747][SQL] Makes GetMapValue nullability more precise

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/complexTypeExtractors.scala

@@ -381,7 +381,21 @@ case class GetMapValue(child: Expression, key: Expression)
   override def right: Expression = key
 
   /** `Null` is returned for invalid ordinals. */
-  override def nullable: Boolean = true
+  override def nullable: Boolean = if (key.foldable && !key.nullable) {
+    val keyObj = key.eval()
+    child match {
+      case m: CreateMap if m.resolved =>
+        m.keys.zip(m.values).filter { case (k, _) => k.foldable && !k.nullable }.find {
+          case (k, _) if k.eval() == keyObj => true
+          case _ => false
+        }.map(_._2.nullable).getOrElse(true)
+      case _ =>
+        true
+    }
+  } else {
+    true
+  }
+
 
   override def dataType: DataType = child.dataType.asInstanceOf[MapType].valueType
 

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

@@ -110,6 +110,25 @@ class ComplexTypeSuite extends SparkFunSuite with ExpressionEvalHelper {
     checkEvaluation(GetMapValue(nestedMap, Literal("a")), Map("b" -> "c"))
   }
 
+  test("SPARK-26747 handles GetMapValue nullability correctly when input key is foldable") {
+    // String key test
+    val k1 = Literal("k1")
+    val v1 = AttributeReference("v1", StringType, nullable = true)()
+    val k2 = Literal("k2")
+    val v2 = AttributeReference("v2", StringType, nullable = false)()
+    val map1 = CreateMap(k1 :: v1 :: k2 :: v2 :: Nil)
+    assert(GetMapValue(map1, Literal("k1")).nullable)
+    assert(!GetMapValue(map1, Literal("k2")).nullable)
+    assert(GetMapValue(map1, Literal("non-existent-key")).nullable)
+
+    // Complex type key test
+    val k3 = Literal.create((1, "a"))
+    val k4 = Literal.create((2, "b"))
+    val map2 = CreateMap(k3 :: v1 :: k4 :: v2 :: Nil)
+    assert(GetMapValue(map2, Literal.create((1, "a"))).nullable)
+    assert(!GetMapValue(map2, Literal.create((2, "b"))).nullable)
+  }
+
   test("GetStructField") {
     val typeS = StructType(StructField("a", IntegerType) :: Nil)
     val struct = Literal.create(create_row(1), typeS)