planner: refine PhysicalProjection.ResolveIndices during neighbouring Proj

.gitignore

@@ -16,3 +16,4 @@ explain_test
 cmd/explaintest/explain-test.out
 _tools/
 *.fail.go
+parser/

executor/executor_test.go

@@ -1081,6 +1081,13 @@ func (s *testSuite) TestUnion(c *C) {
 	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"))
+
+	tk.MustExec("drop table if exists t1, t2")
+	tk.MustExec("create table t1(a int, b int)")
+	tk.MustExec("create table t2(a int, b int)")
+	tk.MustExec("insert into t1 value(1, 1), (2, 2)")
+	tk.MustExec("insert into t2 value(1, 1), (2, 2)")
+	tk.MustQuery("select sum(c) from (select t1.a as a, t1.a as c, length(t1.b) from t1  union select a, b, b from t2) t;").Check(testkit.Rows("5"))
 }
 
 func (s *testSuite) TestIn(c *C) {

planner/core/logical_plan_builder.go

@@ -701,8 +701,14 @@ func (b *PlanBuilder) buildProjection4Union(u *LogicalUnionAll) {
 	// Process each child and add a projection above original child.
 	// So the schema of `UnionAll` can be the same with its children's.
 	for childID, child := range u.children {
+		childProj, isChildProj := child.(*LogicalProjection)
 		exprs := make([]expression.Expression, len(child.Schema().Columns))
 		for i, srcCol := range child.Schema().Columns {
+			if isChildProj {
+				if exprCol, isCol := childProj.Exprs[i].(*expression.Column); isCol {
+					srcCol.UniqueID = exprCol.UniqueID
+				}
+			}
 			dstType := unionCols[i].RetType
 			srcType := srcCol.RetType
 			if !srcType.Equal(dstType) {