[GitHub] [spark] wangyum commented on a change in pull request #31908: [SPARK-34808][SQL] Removes outer join if it only has DISTINCT on streamed side

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

wangyum commented on a change in pull request #31908:
URL: https://github.com/apache/spark/pull/31908#discussion_r643628381



##########
File path: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/joins.scala
##########
@@ -165,6 +170,23 @@ object EliminateOuterJoin extends Rule[LogicalPlan] with PredicateHelper {
     case f @ Filter(condition, j @ Join(_, _, RightOuter | LeftOuter | FullOuter, _, _)) =>
       val newJoinType = buildNewJoinType(f, j)
       if (j.joinType == newJoinType) f else Filter(condition, j.copy(joinType = newJoinType))
+
+    case a @ Aggregate(_, _, join @ Join(left, _, LeftOuter, _, _))
+        if a.isDistinct && a.references.subsetOf(AttributeSet(left.output)) &&
+          !canPlanAsBroadcastHashJoin(join, conf) =>

Review comment:
       The result may be incorrect if always remove the join. For example:
   ```
   0: jdbc:hive2://hdc49-mcc10-01-0510-2005-006-> create table test11.t1 using parquet as select id % 3 as a, id as b from range(10);
   +---------+--+
   | Result  |
   +---------+--+
   +---------+--+
   No rows selected (1.611 seconds)
   0: jdbc:hive2://hdc49-mcc10-01-0510-2005-006-> create table test11.t2 using parquet as select id % 3 as x, id as y from range(5);
   +---------+--+
   | Result  |
   +---------+--+
   +---------+--+
   No rows selected (1.043 seconds)
   0: jdbc:hive2://hdc49-mcc10-01-0510-2005-006-> select t1.a from t1 left join t2 on a = x;
   +----+--+
   | a  |
   +----+--+
   | 0  |
   | 0  |
   | 1  |
   | 1  |
   | 0  |
   | 0  |
   | 1  |
   | 1  |
   | 2  |
   | 0  |
   | 0  |
   | 1  |
   | 1  |
   | 2  |
   | 0  |
   | 0  |
   | 2  |
   +----+--+
   17 rows selected (1.409 seconds)
   ```




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