expression: make sysdate unfoldable

cmd/explaintest/r/select.result

@@ -328,3 +328,7 @@ Point_Get_1	1.00	root	table:t, handle:1
 desc select * from t where a = '1';
 id	count	task	operator info
 Point_Get_1	1.00	root	table:t, handle:1
+desc select sysdate(), sleep(1), sysdate();
+id	count	task	operator info
+Projection_3	1.00	root	sysdate(), sleep(1), sysdate()
+└─TableDual_4	1.00	root	rows:1

cmd/explaintest/t/select.test

@@ -163,3 +163,5 @@ drop table if exists t;
 create table t(a bigint primary key, b bigint);
 desc select * from t where a = 1;
 desc select * from t where a = '1';
+
+desc select sysdate(), sleep(1), sysdate();

expression/function_traits.go

@@ -40,6 +40,7 @@ var UnCacheableFunctions = map[string]struct{}{
 
 // unFoldableFunctions stores functions which can not be folded duration constant folding stage.
 var unFoldableFunctions = map[string]struct{}{
+	ast.Sysdate:   {},
 	ast.FoundRows: {},
 	ast.Rand:      {},
 	ast.UUID:      {},

expression/function_traits_test.go

@@ -0,0 +1,26 @@
+// Copyright 2018 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 expression
+
+import (
+	. "github.com/pingcap/check"
+	"github.com/pingcap/tidb/ast"
+	"github.com/pingcap/tidb/util/testleak"
+)
+
+func (s *testEvaluatorSuite) TestUnfoldableFuncs(c *C) {
+	defer testleak.AfterTest(c)()
+	_, ok := unFoldableFunctions[ast.Sysdate]
+	c.Assert(ok, IsTrue)
+}