wankunde commented on code in PR #42450:
URL: https://github.com/apache/spark/pull/42450#discussion_r1299551600


##########
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/codegen/CodegenFallback.scala:
##########
@@ -46,21 +46,54 @@ trait CodegenFallback extends Expression {
     val objectTerm = ctx.freshName("obj")
     val placeHolder = ctx.registerComment(this.toString)
     val javaType = CodeGenerator.javaType(this.dataType)
-    if (nullable) {
-      ev.copy(code = code"""
-        $placeHolder
-        Object $objectTerm = ((Expression) references[$idx]).eval($input);
-        boolean ${ev.isNull} = $objectTerm == null;
-        $javaType ${ev.value} = ${CodeGenerator.defaultValue(this.dataType)};
-        if (!${ev.isNull}) {
-          ${ev.value} = (${CodeGenerator.boxedType(this.dataType)}) 
$objectTerm;
-        }""")
+    val childrenGen = children.map(_.genCode(ctx))
+    val childrenCode = childrenGen.map(_.code).mkString("\n")
+    val childrenParameter = childrenGen.map(_.value).mkString(", ")
+    val clazz = this.getClass.getName
+    if (supportWholeStageCodegen) {
+      if (nullable) {
+        ev.copy(code =
+          code"""
+          $placeHolder
+          ${childrenCode}
+          Object $objectTerm = null;
+          if(${childrenGen.map(_.isNull).mkString(" || ")}) {
+            $objectTerm = null;
+          } else {
+            $objectTerm = (($clazz) 
references[$idx]).nullSafeEval(${childrenParameter});

Review Comment:
   No, all the fallback expressions which contains `nullSafeEval()` are 
supported.
   
   BinaryExpression:
   ```
     override def eval(input: InternalRow): Any = {
       val value1 = left.eval(input)
       if (value1 == null) {
         null
       } else {
         val value2 = right.eval(input)
         if (value2 == null) {
           null
         } else {
           nullSafeEval(value1, value2)
         }
       }
     }
   ```
   TernaryExpression:
   ```scala
     override def eval(input: InternalRow): Any = {
       val value1 = first.eval(input)
       if (value1 != null) {
         val value2 = second.eval(input)
         if (value2 != null) {
           val value3 = third.eval(input)
           if (value3 != null) {
             return nullSafeEval(value1, value2, value3)
           }
         }
       }
       null
     }
   ```
   and some others.



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