Add test for hash join flatten fix

Author: royi-luo

src/planner/plan/plan_subquery.cpp

@@ -230,7 +230,6 @@ void Planner::planRegularMatch(const QueryGraphCollection& queryGraphCollection,
                 [&joinNodeIDs](LogicalPlan& leftPlan, LogicalPlan& rightPlan) {
                     return CostModel::computeHashJoinCost(joinNodeIDs, leftPlan, rightPlan);
                 });
-            // appendHashJoin(joinNodeIDs, JoinType::INNER, leftPlan, *rightPlan, leftPlan);
         }
     }
     for (auto& predicate : predicatesToPullUp) {

test/test_files/agg/multi_query_part.test

@@ -96,3 +96,13 @@ Bob|Dan|3
 Dan|Alice|3
 Dan|Bob|3
 Dan|Carol|3
+
+# If the aggregation happens on the build side of the join
+# A flatten will needed to be appended to the probe side
+# Otherwise we will have the incorrect number of results
+-LOG GroupByMultiQueryTest8
+-STATEMENT MATCH (a:person)-[k1:knows]->(b:person) WITH a, b, COUNT(*) AS s
+           MATCH (c:person)-[k2:knows]->(a) WITH c, s
+           MATCH (c) WHERE c.ID = 0 RETURN COUNT(*)
+---- 1
+9