From 35e15acf24bebca2d915f423158d4bb73a156dae Mon Sep 17 00:00:00 2001
From: Feilong Liu <feilongliu@meta.com>
Date: Thu, 5 Jun 2025 12:35:33 -0700
Subject: [PATCH] Add comment for AddDistinctForSemiJoinBuild optimizer

---
 .../rule/AddDistinctForSemiJoinBuild.java     | 23 +++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/presto-main-base/src/main/java/com/facebook/presto/sql/planner/iterative/rule/AddDistinctForSemiJoinBuild.java b/presto-main-base/src/main/java/com/facebook/presto/sql/planner/iterative/rule/AddDistinctForSemiJoinBuild.java
index fd6a471f7e078..2f30a29b5af69 100644
--- a/presto-main-base/src/main/java/com/facebook/presto/sql/planner/iterative/rule/AddDistinctForSemiJoinBuild.java
+++ b/presto-main-base/src/main/java/com/facebook/presto/sql/planner/iterative/rule/AddDistinctForSemiJoinBuild.java
@@ -34,6 +34,29 @@
 import static com.facebook.presto.spi.plan.AggregationNode.singleGroupingSet;
 import static com.facebook.presto.sql.planner.plan.Patterns.semiJoin;
 
+/**
+ * Add a distinct aggregation under the build side of semi join, for example:
+ * Rewrite query from
+ * <pre>
+ *     - SemiJoin
+ *          l.col in r.col
+ *          - scan l
+ *              col
+ *          - scan r
+ *              col
+ * </pre>
+ * into
+ * <pre>
+ *     - SemiJoin
+ *          l.col in r.col
+ *          - scan l
+ *              col
+ *          - Aggregate
+ *              group by r.col
+ *              - scan r
+ *                  col
+ * </pre>
+ */
 public class AddDistinctForSemiJoinBuild
         implements Rule<SemiJoinNode>
 {