expression: fix typo (pingcap#4958)

expression/schema.go

@@ -70,7 +70,6 @@ func (s *Schema) Clone() *Schema {
 	}
 	schema := NewSchema(cols...)
 	schema.SetUniqueKeys(keys)
-	schema.TblID2Handle = make(map[int64][]*Column)
 	for id, cols := range s.TblID2Handle {
 		schema.TblID2Handle[id] = make([]*Column, 0, len(cols))
 		for _, col := range cols {
@@ -116,8 +115,8 @@ func (s *Schema) FindColumnAndIndex(astCol *ast.ColumnName) (*Column, int, error
 				idx = i
 			} else {
 				// For query like:
-				// create t1(a int); create t2(d int);
-				// select 1 from t1, t2 where 1 = (select d from t2 where a > 1) where d = 1;
+				// create table t1(a int); create table t2(d int);
+				// select 1 from t1, t2 where 1 = (select d from t2 where a > 1) and d = 1;
 				// we will get an Apply operator whose schema is [test.t1.a, test.t2.d, test.t2.d],
 				// we check whether the column of the schema comes from a subquery to avoid
 				// causing the ambiguous error when resolve the column `d` in the Selection.