planner: fix the issue about simplify outer join to inner join (#51750)…

… (#52079)

close #51560

planner/core/issuetest/BUILD.bazel

@@ -6,6 +6,6 @@ go_test(
     srcs = ["planner_issue_test.go"],
     flaky = True,
     race = "on",
-    shard_count = 10,
+    shard_count = 11,
     deps = ["//testkit"],
 )

planner/core/issuetest/planner_issue_test.go

@@ -217,6 +217,20 @@ func TestIssue48969(t *testing.T) {
 	tk.MustQuery("select * from test2").Check(testkit.Rows("1 0", "2 0", "3 0", "4 4", "5 5"))
 }
 
+func TestIssue51670(t *testing.T) {
+	store := testkit.CreateMockStore(t)
+	tk := testkit.NewTestKit(t, store)
+	tk.MustExec("use test")
+	tk.MustExec("create table A(a int primary key, b int);")
+	tk.MustExec("create table B(b int primary key);")
+	tk.MustExec("create table C(c int primary key, b int);")
+	tk.MustExec("insert into A values (2, 1), (3, 2);")
+	tk.MustExec("insert into B values (1), (2);")
+	// The two should return the same result set.
+	tk.MustQuery("select b.b from A a left join (B b left join C c on b.b = c.b) on b.b = a.b where a.a in (2, 3);").Sort().Check(testkit.Rows("1", "2"))
+	tk.MustQuery("select b.b from A a left join (B b left join C c on b.b = c.b) on b.b = a.b where a.a in (2, 3, null);").Sort().Check(testkit.Rows("1", "2"))
+}
+
 func TestIssue50614(t *testing.T) {
 	store := testkit.CreateMockStore(t)
 	tk := testkit.NewTestKit(t, store)

planner/core/rule_predicate_push_down.go

@@ -394,14 +394,6 @@ func simplifyOuterJoin(p *LogicalJoin, predicates []expression.Expression) {
 		innerTable, outerTable = outerTable, innerTable
 	}
 
-	// first simplify embedded outer join.
-	if innerPlan, ok := innerTable.(*LogicalJoin); ok {
-		simplifyOuterJoin(innerPlan, predicates)
-	}
-	if outerPlan, ok := outerTable.(*LogicalJoin); ok {
-		simplifyOuterJoin(outerPlan, predicates)
-	}
-
 	if p.JoinType == InnerJoin {
 		return
 	}