executor, planner: clone proj schema for different children in buildP…

…roj4Union (pingcap#7999)

executor/executor_test.go

@@ -1035,6 +1035,11 @@ func (s *testSuite) TestUnion(c *C) {
 	tk.MustExec(`set @@tidb_max_chunk_size=2;`)
 	tk.MustQuery(`select count(*) from (select t1.a, t1.b from t1 left join t2 on t1.a=t2.a union all select t1.a, t1.a from t1 left join t2 on t1.a=t2.a) tmp;`).Check(testkit.Rows("128"))
 	tk.MustQuery(`select tmp.a, count(*) from (select t1.a, t1.b from t1 left join t2 on t1.a=t2.a union all select t1.a, t1.a from t1 left join t2 on t1.a=t2.a) tmp;`).Check(testkit.Rows("1 128"))
+
+	tk.MustExec("drop table if exists t")
+	tk.MustExec("create table t(a int, b int)")
+	tk.MustExec("insert into t value(1 ,2)")
+	tk.MustQuery("select a, b from (select a, 0 as d, b from t union all select a, 0 as d, b from t) test;").Check(testkit.Rows("1 2", "1 2"))
 }
 
 func (s *testSuite) TestIn(c *C) {

planner/core/logical_plan_builder.go

@@ -678,7 +678,7 @@ func (b *planBuilder) buildProjection4Union(u *LogicalUnionAll) {
 		}
 		b.optFlag |= flagEliminateProjection
 		proj := LogicalProjection{Exprs: exprs}.init(b.ctx)
-		proj.SetSchema(expression.NewSchema(unionCols...))
+		proj.SetSchema(u.schema.Clone())
 		proj.SetChildren(child)
 		u.children[childID] = proj
 	}