frankyin-factual commented on a change in pull request #28898:
URL: https://github.com/apache/spark/pull/28898#discussion_r448758676



##########
File path: 
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/NestedColumnAliasing.scala
##########
@@ -172,13 +188,23 @@ object NestedColumnAliasing {
           (f, Alias(f, s"_gen_alias_${exprId.id}")(exprId, Seq.empty, None))
         }
 
+
+        // Do deduplication based on semanticEquals, and then sum.
+        val nestedFieldNum = nestedFieldToAlias
+          .foldLeft(Seq[ExtractValue]()) {
+            (unique, curr) => if (!unique.exists(curr._1.semanticEquals(_))) {
+              curr._1 +: unique
+            } else {
+              unique
+            }
+          }
+          .map { t => totalFieldNum(t.dataType)  }
+          .sum

Review comment:
       Hmm, I don't quite follow. All I change is to not double-counting 
duplicated nested columns. The counting method doesn't even care about the 
expression itself. 




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