viirya commented on a change in pull request #29812:
URL: https://github.com/apache/spark/pull/29812#discussion_r492425468



##########
File path: 
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/WithFields.scala
##########
@@ -17,16 +17,29 @@
 
 package org.apache.spark.sql.catalyst.optimizer
 
-import org.apache.spark.sql.catalyst.expressions.WithFields
+import scala.collection.mutable
+
+import org.apache.spark.sql.catalyst.expressions.{Expression, GetStructField, 
WithFields}
 import org.apache.spark.sql.catalyst.plans.logical.LogicalPlan
 import org.apache.spark.sql.catalyst.rules.Rule
 
 
 /**
- * Combines all adjacent [[WithFields]] expression into a single 
[[WithFields]] expression.
+ * Optimizes [[WithFields]] expression chains.
  */
-object CombineWithFields extends Rule[LogicalPlan] {
+object OptimizeWithFields extends Rule[LogicalPlan] {
   def apply(plan: LogicalPlan): LogicalPlan = plan transformAllExpressions {
+    case WithFields(structExpr, names, values) if names.distinct.length != 
names.length =>
+      val newNames = mutable.ArrayBuffer.empty[String]
+      val newValues = mutable.ArrayBuffer.empty[Expression]
+      names.zip(values).reverse.foreach { case (name, value) =>
+        if (!newNames.contains(name)) {
+          newNames += name
+          newValues += value
+        }
+      }
+      WithFields(structExpr, names = newNames.reverse.toSeq, valExprs = 
newValues.reverse.toSeq)

Review comment:
       You are right. It is eventually the same. This is coming from improving 
scalability of #29587. This is applied during I fixed the scalability issue. I 
found this is useful to reduce the complex of `WithFields` expression tree.
   
   I will run these rules in #29587 to simplify expression tree before 
optimizer.




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