executor, plan: eliminate projection if the schema len is 0 (pingcap#…

…6318)

executor/executor_test.go

@@ -942,6 +942,13 @@ func (s *testSuite) TestUnion(c *C) {
 	tk.MustExec("insert into t value ('2017-01-01'), ('2017-01-02')")
 	r = tk.MustQuery("(select a from t where a < 0) union (select a from t where a > 0) order by a")
 	r.Check(testkit.Rows("2017-01-01", "2017-01-02"))
+
+	tk.MustExec("drop table if exists t")
+	tk.MustExec("create table t(a int)")
+	tk.MustExec("insert into t value(0),(0)")
+	tk.MustQuery("select 1 from (select a from t union all select a from t) tmp").Check(testkit.Rows("1", "1", "1", "1"))
+	tk.MustQuery("select 1 from (select a from t limit 1 union all select a from t limit 1) tmp").Check(testkit.Rows("1", "1"))
+	tk.MustQuery("select count(1) from (select a from t union all select a from t) tmp").Check(testkit.Rows("4"))
 }
 
 func (s *testSuite) TestIn(c *C) {

plan/eliminate_projection.go

@@ -32,6 +32,9 @@ func canProjectionBeEliminatedLoose(p *LogicalProjection) bool {
 // canProjectionBeEliminatedStrict checks whether a projection can be eliminated, returns true if
 // the projection just copy its child's output.
 func canProjectionBeEliminatedStrict(p *PhysicalProjection) bool {
+	if p.Schema().Len() == 0 {
+		return true
+	}
 	child := p.Children()[0]
 	if p.Schema().Len() != child.Schema().Len() {
 		return false