[GitHub] [spark] rxin commented on a change in pull request #24271: [SPAR-27342][SQL] Optimize Limit 0 queries

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rxin commented on a change in pull request #24271: [SPAR-27342][SQL] Optimize Limit 0 queries
URL: https://github.com/apache/spark/pull/24271#discussion_r271505321
 
 

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 File path: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/PropagateEmptyRelation.scala
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 @@ -108,5 +108,27 @@ object PropagateEmptyRelation extends Rule[LogicalPlan] with PredicateHelper wit
       case Generate(_: Explode, _, _, _, _, _) => empty(p)
       case _ => p
     }
+
+    // Nodes below GlobalLimit or LocalLimit can be pruned if the limit value is zero (0).
+    // Any subtree in the logical plan that has GlobalLimit 0 or LocalLimit 0 as its root is
+    // semantically equivalent to an empty relation.
+    //
+    // In such cases, the effects of Limit 0 can be propagated through the Logical Plan by replacing
+    // the (Global/Local) Limit subtree with an empty LocalRelation, thereby pruning the subtree
+    // below and triggering other optimization rules of PropagateEmptyRelation to propagate the
+    // changes up the Logical Plan.
+    //
+    // Replace Global Limit 0 nodes with empty Local Relation
+    case p @ GlobalLimit(IntegerLiteral(limit), _) if limit == 0 =>
 
 Review comment:
   it's better to do a new rule, to make it more modular.
   

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