[GitHub] [spark] dongjoon-hyun commented on a change in pull request #24637: [SPARK-27707][SQL] Prune unnecessary nested fields from Generate in explode

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dongjoon-hyun commented on a change in pull request #24637: [SPARK-27707][SQL] Prune unnecessary nested fields from Generate in explode
URL: https://github.com/apache/spark/pull/24637#discussion_r300880428
 
 

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 File path: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala
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 @@ -588,6 +588,16 @@ object ColumnPruning extends Rule[LogicalPlan] {
         .map(_._2)
       p.copy(child = g.copy(child = newChild, unrequiredChildIndex = unrequiredIndices))
 
+    // prune unrequired nested fields
+    case p @ Project(projectList, g: Generate) if SQLConf.get.nestedSchemaPruningEnabled =>
+      NestedColumnAliasing.getAliasSubMap(projectList).map {
+        case (nestedFieldToAlias, attrToAliases) =>
+          val newChild = g.withNewChildren(g.children.map { child =>
+            Project(child.output ++ attrToAliases.values.flatten, child)
 
 Review comment:
   This looks addition instead of a replacing. So, this may optimize `explode`, but we cannot avoid the full scan of underlying parquet files?

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