plan: selection to dual directly if condition is always false. (#4980)

plan/logical_plan_builder.go

@@ -407,8 +407,13 @@ func (b *planBuilder) buildSelection(p LogicalPlan, where ast.ExprNode, AggMappe
 		for _, item := range cnfItems {
 			if con, ok := item.(*expression.Constant); ok {
 				ret, err := expression.EvalBool(expression.CNFExprs{con}, nil, b.ctx)
-				if ret || err != nil {
+				if err != nil || ret {
 					continue
+				} else {
+					// If there is condition which is always false, return dual plan directly.
+					dual := TableDual{}.init(b.allocator, b.ctx)
+					dual.SetSchema(p.Schema().Clone())
+					return dual
 				}
 			}
 			expressions = append(expressions, item)

plan/logical_plan_test.go

@@ -549,7 +549,7 @@ func (s *testPlanSuite) TestPredicatePushDown(c *C) {
 		},
 		{
 			sql:  "select * from (select a, sum(b) as s from t group by a having 1 = 0) k where a > 1",
-			best: "DataScan(t)->Selection->Aggr(sum(test.t.b),firstrow(test.t.a))->Selection->Projection->Projection",
+			best: "Dual->Selection->Projection",
 		},
 		{
 			sql:  "select a, count(a) cnt from t group by a having cnt < 1",
@@ -678,6 +678,10 @@ func (s *testPlanSuite) TestPlanBuilder(c *C) {
 			sql:  "select substr(\"abc\", 1)",
 			plan: "Dual->Projection",
 		},
+		{
+			sql:  "select * from t t1, t t2 where 1 = 0",
+			plan: "Dual->Projection",
+		},
 	}
 	for _, ca := range tests {
 		comment := Commentf("for %s", ca.sql)

plan/physical_plan_test.go

@@ -418,7 +418,7 @@ func (s *testPlanSuite) TestCBO(c *C) {
 		},
 		{
 			sql:  "select count(*) from t t1 having 1 = 0",
-			best: "Dual->HashAgg->Selection",
+			best: "Dual",
 		},
 		{
 			sql:  "select sum(a.b), sum(b.b) from t a join t b on a.c = b.c group by a.d order by a.d",