diff --git a/executor/builder.go b/executor/builder.go
index f1e2e689f5826..d192cc691fcfd 100644
--- a/executor/builder.go
+++ b/executor/builder.go
@@ -2080,7 +2080,13 @@ func (b *executorBuilder) buildIndexLookUpJoin(v *plannercore.PhysicalIndexJoin)
 	innerPlan := v.Children()[v.InnerChildIdx]
 	innerTypes := make([]*types.FieldType, innerPlan.Schema().Len())
 	for i, col := range innerPlan.Schema().Columns {
-		innerTypes[i] = col.RetType
+		innerTypes[i] = col.RetType.Clone()
+		// The `innerTypes` would be called for `Datum.ConvertTo` when converting the columns from outer table
+		// to build hash map or construct lookup keys. So we need to modify its Flen otherwise there would be
+		// truncate error. See issue https://github.com/pingcap/tidb/issues/21232 for example.
+		if innerTypes[i].EvalType() == types.ETString {
+			innerTypes[i].Flen = types.UnspecifiedLength
+		}
 	}
 
 	var (
@@ -2169,7 +2175,13 @@ func (b *executorBuilder) buildIndexLookUpMergeJoin(v *plannercore.PhysicalIndex
 	innerPlan := v.Children()[v.InnerChildIdx]
 	innerTypes := make([]*types.FieldType, innerPlan.Schema().Len())
 	for i, col := range innerPlan.Schema().Columns {
-		innerTypes[i] = col.RetType
+		innerTypes[i] = col.RetType.Clone()
+		// The `innerTypes` would be called for `Datum.ConvertTo` when converting the columns from outer table
+		// to build hash map or construct lookup keys. So we need to modify its Flen otherwise there would be
+		// truncate error. See issue https://github.com/pingcap/tidb/issues/21232 for example.
+		if innerTypes[i].EvalType() == types.ETString {
+			innerTypes[i].Flen = types.UnspecifiedLength
+		}
 	}
 	var (
 		outerFilter           []expression.Expression
diff --git a/executor/write_test.go b/executor/write_test.go
index eb4234ba4dd97..1e9c95c3f336f 100644
--- a/executor/write_test.go
+++ b/executor/write_test.go
@@ -2872,3 +2872,22 @@ func (s *testSerialSuite1) TestIssue20724(c *C) {
 	tk.MustQuery("select * from t1").Check(testkit.Rows("A"))
 	tk.MustExec("drop table t1")
 }
+
+func (s *testSuite) TestIssue21232(c *C) {
+	tk := testkit.NewTestKit(c, s.store)
+	tk.MustExec("use test")
+	tk.MustExec("drop table if exists t, t1")
+	tk.MustExec("create table t(a varchar(1), index idx(a))")
+	tk.MustExec("create table t1(a varchar(5), index idx(a))")
+	tk.MustExec("insert into t values('a'), ('b')")
+	tk.MustExec("insert into t1 values('a'), ('bbbbb')")
+	tk.MustExec("update /*+ INL_JOIN(t) */ t, t1 set t.a='a' where t.a=t1.a")
+	tk.MustQuery("show warnings").Check(testkit.Rows())
+	tk.MustQuery("select * from t").Check(testkit.Rows("a", "b"))
+	tk.MustExec("update /*+ INL_HASH_JOIN(t) */ t, t1 set t.a='a' where t.a=t1.a")
+	tk.MustQuery("show warnings").Check(testkit.Rows())
+	tk.MustQuery("select * from t").Check(testkit.Rows("a", "b"))
+	tk.MustExec("update /*+ INL_MERGE_JOIN(t) */ t, t1 set t.a='a' where t.a=t1.a")
+	tk.MustQuery("show warnings").Check(testkit.Rows())
+	tk.MustQuery("select * from t").Check(testkit.Rows("a", "b"))
+}