From 3c413a7e50536a29416d9bd4853447e20780f0bf Mon Sep 17 00:00:00 2001
From: Josipmrden <josip.mrden@memgraph.io>
Date: Sun, 12 Nov 2023 20:45:02 +0100
Subject: [PATCH] Fix hash join expression matching (#1496)

---
 src/query/plan/operator.cpp                        |  4 ++--
 src/query/plan/rewrite/join.hpp                    |  8 ++++++++
 .../tests/memgraph_V1/features/cartesian.feature   | 14 ++++++++++++++
 3 files changed, 24 insertions(+), 2 deletions(-)

diff --git a/src/query/plan/operator.cpp b/src/query/plan/operator.cpp
index 69748014e..792d278e8 100644
--- a/src/query/plan/operator.cpp
+++ b/src/query/plan/operator.cpp
@@ -5404,7 +5404,7 @@ class HashJoinCursor : public Cursor {
         // Check if the join value from the pulled frame is shared with any left frames
         ExpressionEvaluator evaluator(&frame, context.symbol_table, context.evaluation_context, context.db_accessor,
                                       storage::View::OLD);
-        auto right_value = self_.hash_join_condition_->expression1_->Accept(evaluator);
+        auto right_value = self_.hash_join_condition_->expression2_->Accept(evaluator);
         if (hashtable_.contains(right_value)) {
           // If so, finish pulling for now and proceed to joining the pulled frame
           right_op_frame_.assign(frame.elems().begin(), frame.elems().end());
@@ -5452,7 +5452,7 @@ class HashJoinCursor : public Cursor {
     while (left_op_cursor_->Pull(frame, context)) {
       ExpressionEvaluator evaluator(&frame, context.symbol_table, context.evaluation_context, context.db_accessor,
                                     storage::View::OLD);
-      auto left_value = self_.hash_join_condition_->expression2_->Accept(evaluator);
+      auto left_value = self_.hash_join_condition_->expression1_->Accept(evaluator);
       if (left_value.type() != TypedValue::Type::Null) {
         hashtable_[left_value].emplace_back(frame.elems().begin(), frame.elems().end());
       }
diff --git a/src/query/plan/rewrite/join.hpp b/src/query/plan/rewrite/join.hpp
index c16f5b60d..5ed2bb090 100644
--- a/src/query/plan/rewrite/join.hpp
+++ b/src/query/plan/rewrite/join.hpp
@@ -27,6 +27,7 @@
 
 #include "query/plan/operator.hpp"
 #include "query/plan/preprocess.hpp"
+#include "utils/algorithm.hpp"
 
 namespace memgraph::query::plan {
 
@@ -523,6 +524,13 @@ class JoinRewriter final : public HierarchicalLogicalOperatorVisitor {
       auto rhs_property = rhs_lookup->property_;
       filter_exprs_for_removal_.insert(filter.expression);
       filters_.EraseFilter(filter);
+
+      if (utils::Contains(right_symbols, lhs_symbol) && utils::Contains(left_symbols, rhs_symbol)) {
+        // We need to duplicate this because expressions are shared between plans
+        join_condition = join_condition->Clone(ast_storage_);
+        std::swap(join_condition->expression1_, join_condition->expression2_);
+      }
+
       return std::make_unique<HashJoin>(left_op, left_symbols, right_op, right_symbols, join_condition);
     }
 
diff --git a/tests/gql_behave/tests/memgraph_V1/features/cartesian.feature b/tests/gql_behave/tests/memgraph_V1/features/cartesian.feature
index 809a3d73a..b11d83f0e 100644
--- a/tests/gql_behave/tests/memgraph_V1/features/cartesian.feature
+++ b/tests/gql_behave/tests/memgraph_V1/features/cartesian.feature
@@ -186,6 +186,20 @@ Feature: Cartesian
             | a            | b            |
             | (:A {id: 1}) | (:B {id: 1}) |
 
+    Scenario: Multiple match with WHERE x = y 01 reversed
+        Given an empty graph
+        And having executed
+            """
+            CREATE (:A {id: 1}), (:A {id: 2}), (:B {id: 1})
+            """
+        When executing query:
+            """
+            MATCH (a:A) MATCH (b:B) WHERE b.id = a.id RETURN a, b
+            """
+        Then the result should be:
+            | a            | b            |
+            | (:A {id: 1}) | (:B {id: 1}) |
+
     Scenario: Multiple match with WHERE x = y 02
         Given an empty graph
         And having executed