expression: fix wrong flen for CastAsString funtion

expression/builtin_cast.go

@@ -1836,6 +1836,9 @@ func BuildCastFunction(ctx sessionctx.Context, expr Expression, tp *types.FieldT
 	case types.ETJson:
 		fc = &castAsJSONFunctionClass{baseFunctionClass{ast.Cast, 1, 1}, tp}
 	case types.ETString:
+		if tp.Flen == types.UnspecifiedLength || tp.Flen == types.ErrorLength {
+			tp.Flen = expr.GetType().Flen
+		}
 		fc = &castAsStringFunctionClass{baseFunctionClass{ast.Cast, 1, 1}, tp}
 	}
 	f, err := fc.getFunction(ctx, []Expression{expr})

expression/integration_test.go

@@ -10475,3 +10475,16 @@ func (s *testIntegrationSuite) TestIssue29244(c *C) {
 	tk.MustExec("set tidb_enable_vectorized_expression = off;")
 	tk.MustQuery("select microsecond(a) from t;").Check(testkit.Rows("123500", "123500"))
 }
+
+func (s *testIntegrationSuite) TestIssue29513(c *C) {
+	tk := testkit.NewTestKit(c, s.store)
+	tk.MustExec("use test")
+	tk.MustQuery("select '123' union select cast(45678 as char);").Sort().Check(testkit.Rows("123", "45678"))
+	tk.MustQuery("select '123' union select cast(45678 as char(2));").Sort().Check(testkit.Rows("123", "45"))
+
+	tk.MustExec("drop table if exists t")
+	tk.MustExec("create table t(a int);")
+	tk.MustExec("insert into t values(45678);")
+	tk.MustQuery("select '123' union select cast(a as char) from t;").Sort().Check(testkit.Rows("123", "45678"))
+	tk.MustQuery("select '123' union select cast(a as char(2)) from t;").Sort().Check(testkit.Rows("123", "45"))
+}