[GitHub] [spark] sigmod commented on a change in pull request #35864: [SPARK-38531][SQL] Fix the condition of "Prune unrequired child index" branch of ColumnPruning

[GitBox <gi...@apache.org>]

sigmod commented on a change in pull request #35864:
URL: https://github.com/apache/spark/pull/35864#discussion_r828505854



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File path: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala
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@@ -831,8 +831,16 @@ object ColumnPruning extends Rule[LogicalPlan] {
       e.copy(child = prunedChild(child, e.references))
 
     // prune unrequired references
-    case p @ Project(_, g: Generate) if p.references != g.outputSet =>
-      val requiredAttrs = p.references -- g.producedAttributes ++ g.generator.references
+    // There are 2 types of pruning here:
+    // 1. For attributes in g.child.outputSet that is not used by the generator nor the project,
+    //    we directly remove it from the output list of g.child.
+    // 2. For attributes that is not used by the project but it is used by the generator, we put
+    //    it in g.unrequiredChildIndex to save memory usage.
+    case p @ Project(_, g: Generate) if g.child.output.zipWithIndex.exists(
+      pair =>
+      !p.references.contains(pair._1) &&
+        (!g.generator.references.contains(pair._1) || !g.unrequiredChildIndex.contains(pair._2))) =>

Review comment:
       Does it work if we switch the order case branches? Since the rule is a fix-point batch, it can hit `GeneratorNestedColumnAliasing` in the first iteration and Project(_, g: Generate) in the next?
   
   ```
       // prune unrequired nested fields from `Generate`.
       // prioritize this case branch over  Project(_, g: Generate) for SPARK-38531
       case GeneratorNestedColumnAliasing(rewrittenPlan) => rewrittenPlan
       
      // prune unrequired references
       case p @ Project(_, g: Generate) if p.references != g.outputSet => ...
   ```




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