cloud-fan commented on a change in pull request #27488: 
[SPARK-26580][SQL][ML][FOLLOW-UP] Throw exception when use untyped UDF by 
default
URL: https://github.com/apache/spark/pull/27488#discussion_r376306898
 
 

 ##########
 File path: sql/core/src/main/scala/org/apache/spark/sql/functions.scala
 ##########
 @@ -4732,6 +4733,15 @@ object functions {
    * @since 2.0.0
    */
   def udf(f: AnyRef, dataType: DataType): UserDefinedFunction = {
+    if (!SQLConf.get.getConf(SQLConf.LEGACY_USE_UNTYPED_UDF)) {
+      val errorMsg = "You're using untyped udf, which does not have the input 
type information. " +
+        "So, Spark may blindly pass null to the Scala closure with 
primitive-type argument, " +
+        "and the closure will see the default value of the Java type for the 
null argument, " +
+        "e.g. `udf((x: Int) => x, IntegerType)`, the result is 0 for null 
input. You could use " +
+        "other typed udf APIs to avoid this problem, or set " +
+        "spark.sql.legacy.useUnTypedUdf.enabled to true to insistently use 
this."
 
 Review comment:
   let's not hardcode config names.

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