planner, executor: set new child after injecting Project operator

executor/join_test.go

@@ -1368,3 +1368,16 @@ func (s *testSuite2) TestScalarFuncNullSemiJoin(c *C) {
 	tk.MustExec("insert into s values(null, 1)")
 	tk.MustQuery("select a in (select a+b from s) from t").Check(testkit.Rows("<nil>", "<nil>"))
 }
+
+func (s *testSuite2) TestInjectProjOnTopN(c *C) {
+	tk := testkit.NewTestKit(c, s.store)
+	tk.MustExec("use test")
+	tk.MustExec("drop table if exists t1")
+	tk.MustExec("drop table if exists t2")
+	tk.MustExec("create table t1(a bigint, b bigint)")
+	tk.MustExec("create table t2(a bigint, b bigint)")
+	tk.MustExec("insert into t1 values(1, 1)")
+	tk.MustQuery("select t1.a+t1.b as result from t1 left join t2 on 1 = 0 order by result limit 20;").Check(testkit.Rows(
+		"2",
+	))
+}

planner/core/physical_plan_test.go

@@ -115,7 +115,7 @@ func (s *testPlanSuite) TestDAGPlanBuilderSimpleCase(c *C) {
 		// Test TopN push down in table single read.
 		{
 			sql:  "select c from t order by t.a + t.b limit 1",
-			best: "TableReader(Table(t)->TopN([plus(test.t.a, test.t.b)],0,1))->Projection->TopN([col_3],0,1)->Projection",
+			best: "TableReader(Table(t)->TopN([plus(test.t.a, test.t.b)],0,1))->Projection->TopN([col_3],0,1)->Projection->Projection",
 		},
 		// Test Limit push down in table single read.
 		{
@@ -1219,7 +1219,7 @@ func (s *testPlanSuite) TestAggEliminater(c *C) {
 		// If max/min contains scalar function, we can still do transformation.
 		{
 			sql:  "select max(a+1) from t;",
-			best: "TableReader(Table(t)->Sel([not(isnull(plus(test.t.a, 1)))])->TopN([plus(test.t.a, 1) true],0,1))->Projection->TopN([col_1 true],0,1)->Projection->StreamAgg",
+			best: "TableReader(Table(t)->Sel([not(isnull(plus(test.t.a, 1)))])->TopN([plus(test.t.a, 1) true],0,1))->Projection->TopN([col_1 true],0,1)->Projection->Projection->StreamAgg",
 		},
 		// Do nothing to max+min.
 		{

planner/core/rule_inject_extra_projection.go

@@ -39,8 +39,9 @@ func NewProjInjector() *projInjector {
 }
 
 func (pe *projInjector) inject(plan PhysicalPlan) PhysicalPlan {
-	for _, child := range plan.Children() {
-		pe.inject(child)
+	for i, child := range plan.Children() {
+		plan.Children()[i] = pe.inject(child)
+		// pe.inject(child)
 	}
 
 	switch p := plan.(type) {
@@ -205,5 +206,9 @@ func injectProjBelowSort(p PhysicalPlan, orderByItems []*ByItems) PhysicalPlan {
 	if origChildProj, isChildProj := childPlan.(*PhysicalProjection); isChildProj {
 		refine4NeighbourProj(bottomProj, origChildProj)
 	}
+
+	if topProj.Schema().Len() == 0 {
+		return p
+	}
 	return topProj
 }