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

… (#52078)

close #51560

pkg/planner/core/rule_predicate_push_down.go

@@ -395,14 +395,6 @@ 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
 	}

tests/integrationtest/r/planner/core/issuetest/planner_issue.result

@@ -241,6 +241,31 @@ id	value
 3	0
 4	4
 5	5
+create table A(a int primary key, b int);
+create table B(b int primary key);
+create table C(c int primary key, b int);
+insert into A values (2, 1), (3, 2);
+insert into B values (1), (2);
+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);
+b
+1
+2
+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);
+b
+1
+2
 drop table if exists t0, t1;
 CREATE TABLE t0(c0 NUMERIC);
 CREATE TABLE t1(c0 NUMERIC);

tests/integrationtest/t/planner/core/issuetest/planner_issue.test

@@ -183,6 +183,32 @@ where test2.id in
 );
 select * from test2;
 
+# https://github.com/pingcap/tidb/issues/51560
+create table A(a int primary key, b int);
+create table B(b int primary key);
+create table C(c int primary key, b int);
+
+insert into A values (2, 1), (3, 2);
+insert into B values (1), (2);
+
+# Returns data as expected
+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);
+
+# Returns the same.
+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);
+
 # https://github.com/pingcap/tidb/issues/49109
 drop table if exists t0, t1;
 CREATE TABLE t0(c0 NUMERIC);