diff --git a/executor/executor_test.go b/executor/executor_test.go
index f57b1376810f3..fd4f0f4d1720b 100644
--- a/executor/executor_test.go
+++ b/executor/executor_test.go
@@ -85,6 +85,7 @@ var _ = Suite(&testSuite1{})
 var _ = Suite(&testSuite2{})
 var _ = Suite(&testSuite3{})
 var _ = Suite(&testBypassSuite{})
+var _ = Suite(&testUpdateSuite{})
 
 type testSuite struct {
 	cluster   *mocktikv.Cluster
diff --git a/executor/update_test.go b/executor/update_test.go
new file mode 100644
index 0000000000000..fda7bc0c35c07
--- /dev/null
+++ b/executor/update_test.go
@@ -0,0 +1,92 @@
+// Copyright 2019 PingCAP, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package executor_test
+
+import (
+	"flag"
+	"fmt"
+
+	. "github.com/pingcap/check"
+	"github.com/pingcap/parser"
+	"github.com/pingcap/tidb/domain"
+	"github.com/pingcap/tidb/kv"
+	"github.com/pingcap/tidb/session"
+	"github.com/pingcap/tidb/store/mockstore"
+	"github.com/pingcap/tidb/store/mockstore/mocktikv"
+	"github.com/pingcap/tidb/util/mock"
+	"github.com/pingcap/tidb/util/testkit"
+	"github.com/pingcap/tidb/util/testleak"
+)
+
+type testUpdateSuite struct {
+	cluster   *mocktikv.Cluster
+	mvccStore mocktikv.MVCCStore
+	store     kv.Storage
+	domain    *domain.Domain
+	*parser.Parser
+	ctx *mock.Context
+}
+
+func (s *testUpdateSuite) SetUpSuite(c *C) {
+	testleak.BeforeTest()
+	s.Parser = parser.New()
+	flag.Lookup("mockTikv")
+	useMockTikv := *mockTikv
+	if useMockTikv {
+		s.cluster = mocktikv.NewCluster()
+		mocktikv.BootstrapWithSingleStore(s.cluster)
+		s.mvccStore = mocktikv.MustNewMVCCStore()
+		store, err := mockstore.NewMockTikvStore(
+			mockstore.WithCluster(s.cluster),
+			mockstore.WithMVCCStore(s.mvccStore),
+		)
+		c.Assert(err, IsNil)
+		s.store = store
+		session.SetSchemaLease(0)
+		session.SetStatsLease(0)
+	}
+	d, err := session.BootstrapSession(s.store)
+	c.Assert(err, IsNil)
+	d.SetStatsUpdating(true)
+	s.domain = d
+}
+
+func (s *testUpdateSuite) TearDownSuite(c *C) {
+	s.domain.Close()
+	s.store.Close()
+	testleak.AfterTest(c, TestLeakCheckCnt)()
+}
+
+func (s *testUpdateSuite) TearDownTest(c *C) {
+	tk := testkit.NewTestKit(c, s.store)
+	tk.MustExec("use test")
+	r := tk.MustQuery("show tables")
+	for _, tb := range r.Rows() {
+		tableName := tb[0]
+		tk.MustExec(fmt.Sprintf("drop table %v", tableName))
+	}
+}
+
+func (s *testUpdateSuite) TestUpdateGenColInTxn(c *C) {
+	tk := testkit.NewTestKit(c, s.store)
+	tk.MustExec("use test")
+	tk.MustExec(`create table t(a bigint, b bigint as (a+1));`)
+	tk.MustExec(`begin;`)
+	tk.MustExec(`insert into t(a) values(1);`)
+	err := tk.ExecToErr(`update t set b=6 where b=2;`)
+	c.Assert(err.Error(), Equals, "[planner:3105]The value specified for generated column 'b' in table 't' is not allowed.")
+	tk.MustExec(`commit;`)
+	tk.MustQuery(`select * from t;`).Check(testkit.Rows(
+		`1 2`))
+}
diff --git a/planner/core/logical_plan_builder.go b/planner/core/logical_plan_builder.go
index 592bc86519e1f..c26d894babcbe 100644
--- a/planner/core/logical_plan_builder.go
+++ b/planner/core/logical_plan_builder.go
@@ -2347,15 +2347,27 @@ func extractTableAsNameForUpdate(p LogicalPlan, asNames map[*model.TableInfo][]*
 			asNames[x.tableInfo] = append(asNames[x.tableInfo], alias)
 		}
 	case *LogicalProjection:
-		if x.calculateGenCols {
-			ds := x.Children()[0].(*DataSource)
-			alias := extractTableAlias(x)
-			if alias != nil {
-				if _, ok := asNames[ds.tableInfo]; !ok {
-					asNames[ds.tableInfo] = make([]*model.CIStr, 0, 1)
-				}
-				asNames[ds.tableInfo] = append(asNames[ds.tableInfo], alias)
+		if !x.calculateGenCols {
+			return
+		}
+
+		ds, isDS := x.Children()[0].(*DataSource)
+		if !isDS {
+			// try to extract the DataSource below a LogicalUnionScan.
+			if us, isUS := x.Children()[0].(*LogicalUnionScan); isUS {
+				ds, isDS = us.Children()[0].(*DataSource)
+			}
+		}
+		if !isDS {
+			return
+		}
+
+		alias := extractTableAlias(x)
+		if alias != nil {
+			if _, ok := asNames[ds.tableInfo]; !ok {
+				asNames[ds.tableInfo] = make([]*model.CIStr, 0, 1)
 			}
+			asNames[ds.tableInfo] = append(asNames[ds.tableInfo], alias)
 		}
 	default:
 		for _, child := range p.Children() {