diff --git a/executor/executor_test.go b/executor/executor_test.go
index 13efa78e0eecb..02961c018c7d5 100644
--- a/executor/executor_test.go
+++ b/executor/executor_test.go
@@ -4428,3 +4428,168 @@ func (s *testSuite1) TestPartitionHashCode(c *C) {
 	}
 	wg.Wait()
 }
+<<<<<<< HEAD
+=======
+
+func (s *testSuite1) TestAlterDefaultValue(c *C) {
+	tk := testkit.NewTestKit(c, s.store)
+	tk.MustExec("use test")
+	tk.MustExec("drop table if exists t1")
+	tk.MustExec("create table t(a int, primary key(a))")
+	tk.MustExec("insert into t(a) values(1)")
+	tk.MustExec("alter table t add column b int default 1")
+	tk.MustExec("alter table t alter b set default 2")
+	tk.MustQuery("select b from t where a = 1").Check(testkit.Rows("1"))
+}
+
+type testClusterTableSuite struct {
+	testSuiteWithCliBase
+	rpcserver  *grpc.Server
+	listenAddr string
+}
+
+func (s *testClusterTableSuite) SetUpSuite(c *C) {
+	s.testSuiteWithCliBase.SetUpSuite(c)
+	s.rpcserver, s.listenAddr = s.setUpRPCService(c, ":0")
+}
+
+func (s *testClusterTableSuite) setUpRPCService(c *C, addr string) (*grpc.Server, string) {
+	sm := &mockSessionManager1{}
+	sm.PS = append(sm.PS, &util.ProcessInfo{
+		ID:      1,
+		User:    "root",
+		Host:    "127.0.0.1",
+		Command: mysql.ComQuery,
+	})
+	lis, err := net.Listen("tcp", addr)
+	c.Assert(err, IsNil)
+	srv := server.NewRPCServer(config.GetGlobalConfig(), s.dom, sm)
+	port := lis.Addr().(*net.TCPAddr).Port
+	addr = fmt.Sprintf("127.0.0.1:%d", port)
+	go func() {
+		err = srv.Serve(lis)
+		c.Assert(err, IsNil)
+	}()
+	cfg := config.GetGlobalConfig()
+	cfg.Status.StatusPort = uint(port)
+	config.StoreGlobalConfig(cfg)
+	return srv, addr
+}
+func (s *testClusterTableSuite) TearDownSuite(c *C) {
+	if s.rpcserver != nil {
+		s.rpcserver.Stop()
+		s.rpcserver = nil
+	}
+	s.testSuiteWithCliBase.TearDownSuite(c)
+}
+
+func (s *testClusterTableSuite) TestSlowQuery(c *C) {
+	writeFile := func(file string, data string) {
+		f, err := os.OpenFile(file, os.O_CREATE|os.O_WRONLY, 0644)
+		c.Assert(err, IsNil)
+		_, err = f.Write([]byte(data))
+		c.Assert(f.Close(), IsNil)
+		c.Assert(err, IsNil)
+	}
+
+	logData0 := ""
+	logData1 := `
+# Time: 2020-02-15T18:00:01.000000+08:00
+select 1;
+# Time: 2020-02-15T19:00:05.000000+08:00
+select 2;`
+	logData2 := `
+# Time: 2020-02-16T18:00:01.000000+08:00
+select 3;
+# Time: 2020-02-16T18:00:05.000000+08:00
+select 4;`
+	logData3 := `
+# Time: 2020-02-16T19:00:00.000000+08:00
+select 5;
+# Time: 2020-02-17T18:00:05.000000+08:00
+select 6;`
+	fileName0 := "tidb-slow-2020-02-14T19-04-05.01.log"
+	fileName1 := "tidb-slow-2020-02-15T19-04-05.01.log"
+	fileName2 := "tidb-slow-2020-02-16T19-04-05.01.log"
+	fileName3 := "tidb-slow.log"
+	writeFile(fileName0, logData0)
+	writeFile(fileName1, logData1)
+	writeFile(fileName2, logData2)
+	writeFile(fileName3, logData3)
+	defer func() {
+		os.Remove(fileName0)
+		os.Remove(fileName1)
+		os.Remove(fileName2)
+		os.Remove(fileName3)
+	}()
+	tk := testkit.NewTestKitWithInit(c, s.store)
+	loc, err := time.LoadLocation("Asia/Shanghai")
+	c.Assert(err, IsNil)
+	tk.Se.GetSessionVars().TimeZone = loc
+	tk.MustExec("use information_schema")
+	cases := []struct {
+		sql    string
+		result []string
+	}{
+		{
+			sql:    "select count(*),min(time),max(time) from %s where time > '2019-01-26 21:51:00' and time < now()",
+			result: []string{"6|2020-02-15 18:00:01.000000|2020-02-17 18:00:05.000000"},
+		},
+		{
+			sql:    "select count(*),min(time),max(time) from %s where time > '2020-02-15 19:00:00' and time < '2020-02-16 18:00:02'",
+			result: []string{"2|2020-02-15 19:00:05.000000|2020-02-16 18:00:01.000000"},
+		},
+		{
+			sql:    "select count(*),min(time),max(time) from %s where time > '2020-02-16 18:00:02' and time < '2020-02-17 17:00:00'",
+			result: []string{"2|2020-02-16 18:00:05.000000|2020-02-16 19:00:00.000000"},
+		},
+		{
+			sql:    "select count(*),min(time),max(time) from %s where time > '2020-02-16 18:00:02' and time < '2020-02-17 20:00:00'",
+			result: []string{"3|2020-02-16 18:00:05.000000|2020-02-17 18:00:05.000000"},
+		},
+		{
+			sql:    "select count(*),min(time),max(time) from %s",
+			result: []string{"2|2020-02-16 19:00:00.000000|2020-02-17 18:00:05.000000"},
+		},
+		{
+			sql:    "select count(*),min(time) from %s where time > '2020-02-16 20:00:00'",
+			result: []string{"1|2020-02-17 18:00:05.000000"},
+		},
+		{
+			sql:    "select count(*) from %s where time > '2020-02-17 20:00:00'",
+			result: []string{"0"},
+		},
+		{
+			sql:    "select query from %s where time > '2019-01-26 21:51:00' and time < now()",
+			result: []string{"select 1;", "select 2;", "select 3;", "select 4;", "select 5;", "select 6;"},
+		},
+	}
+	for _, cas := range cases {
+		sql := fmt.Sprintf(cas.sql, "slow_query")
+		tk.MustQuery(sql).Check(testutil.RowsWithSep("|", cas.result...))
+		sql = fmt.Sprintf(cas.sql, "cluster_slow_query")
+		tk.MustQuery(sql).Check(testutil.RowsWithSep("|", cas.result...))
+	}
+}
+
+func (s *testSuite1) TestIssue15718(c *C) {
+	tk := testkit.NewTestKit(c, s.store)
+	tk.MustExec("use test;")
+	tk.MustExec("drop table if exists tt;")
+	tk.MustExec("create table tt(a decimal(10, 0), b varchar(1), c time);")
+	tk.MustExec("insert into tt values(0, '2', null), (7, null, '1122'), (NULL, 'w', null), (NULL, '2', '3344'), (NULL, NULL, '0'), (7, 'f', '33');")
+	tk.MustQuery("select a and b as d, a or c as e from tt;").Check(testkit.Rows("0 <nil>", "<nil> 1", "0 <nil>", "<nil> 1", "<nil> <nil>", "0 1"))
+}
+
+func (s *testSuite1) TestIssue15767(c *C) {
+	tk := testkit.NewTestKit(c, s.store)
+	tk.MustExec("use test;")
+	tk.MustExec("drop table if exists tt;")
+	tk.MustExec("create table t(a int, b char);")
+	tk.MustExec("insert into t values (1,'s'),(2,'b'),(1,'c'),(2,'e'),(1,'a');")
+	tk.MustExec("insert into t select * from t;")
+	tk.MustExec("insert into t select * from t;")
+	tk.MustExec("insert into t select * from t;")
+	tk.MustQuery("select b, count(*) from ( select b from t order by a limit 20 offset 2) as s group by b order by b;").Check(testkit.Rows("a 6", "c 7", "s 7"))
+}
+>>>>>>> 6a45a7d... mocktikv: avoid sorting multiple times with unstable order (#15898)
diff --git a/store/mockstore/mocktikv/executor.go b/store/mockstore/mocktikv/executor.go
index 7808e18cff554..522b23a2b64e3 100644
--- a/store/mockstore/mocktikv/executor.go
+++ b/store/mockstore/mocktikv/executor.go
@@ -562,6 +562,7 @@ func (e *topNExec) Next(ctx context.Context) (value [][]byte, err error) {
 				return nil, errors.Trace(err)
 			}
 			if !hasMore {
+				sort.Sort(&e.heap.topNSorter)
 				break
 			}
 		}
@@ -570,7 +571,6 @@ func (e *topNExec) Next(ctx context.Context) (value [][]byte, err error) {
 	if e.cursor >= len(e.heap.rows) {
 		return nil, nil
 	}
-	sort.Sort(&e.heap.topNSorter)
 	row := e.heap.rows[e.cursor]
 	e.cursor++