cherry pick pingcap#26561 to release-5.1

Signed-off-by: ti-srebot <ti-srebot@pingcap.com>

planner/core/integration_test.go

@@ -3901,3 +3901,70 @@ func (s *testIntegrationSerialSuite) TestMergeContinuousSelections(c *C) {
 		res.Check(testkit.Rows(output[i].Plan...))
 	}
 }
+<<<<<<< HEAD
+=======
+
+func (s *testIntegrationSerialSuite) TestSelectIgnoreTemporaryTableInView(c *C) {
+	tk := testkit.NewTestKit(c, s.store)
+	tk.MustExec("use test")
+
+	tk.Se.Auth(&auth.UserIdentity{Username: "root", Hostname: "localhost", CurrentUser: true, AuthUsername: "root", AuthHostname: "%"}, nil, []byte("012345678901234567890"))
+	tk.MustExec("set @@tidb_enable_noop_functions=1")
+	tk.MustExec("create table t1 (a int, b int)")
+	tk.MustExec("create table t2 (c int, d int)")
+	tk.MustExec("create view v1 as select * from t1 order by a")
+	tk.MustExec("create view v2 as select * from ((select * from t1) union (select * from t2)) as tt order by a, b")
+	tk.MustExec("create view v3 as select * from v1 order by a")
+	tk.MustExec("create view v4 as select * from t1, t2 where t1.a = t2.c order by a, b")
+	tk.MustExec("create view v5 as select * from (select * from t1) as t1 order by a")
+
+	tk.MustExec("insert into t1 values (1, 2), (3, 4)")
+	tk.MustExec("insert into t2 values (3, 5), (6, 7)")
+
+	tk.MustExec("create temporary table t1 (a int, b int)")
+	tk.MustExec("create temporary table t2 (c int, d int)")
+	tk.MustQuery("select * from t1").Check(testkit.Rows())
+	tk.MustQuery("select * from t2").Check(testkit.Rows())
+
+	tk.MustQuery("select * from v1").Check(testkit.Rows("1 2", "3 4"))
+	tk.MustQuery("select * from v2").Check(testkit.Rows("1 2", "3 4", "3 5", "6 7"))
+	tk.MustQuery("select * from v3").Check(testkit.Rows("1 2", "3 4"))
+	tk.MustQuery("select * from v4").Check(testkit.Rows("3 4 3 5"))
+	tk.MustQuery("select * from v5").Check(testkit.Rows("1 2", "3 4"))
+
+}
+
+func (s *testIntegrationSerialSuite) TestIssue26250(c *C) {
+	tk := testkit.NewTestKit(c, s.store)
+	tk.MustExec("use test")
+	tk.MustExec("create table tp (id int primary key) partition by range (id) (partition p0 values less than (100));")
+	tk.MustExec("create table tn (id int primary key);")
+	tk.MustExec("insert into tp values(1),(2);")
+	tk.MustExec("insert into tn values(1),(2);")
+	tk.MustQuery("select * from tp,tn where tp.id=tn.id and tn.id=1 for update;").Check(testkit.Rows("1 1"))
+}
+
+// https://github.com/pingcap/tidb/issues/26214
+func (s *testIntegrationSerialSuite) TestIssue26214(c *C) {
+	originalVal := config.GetGlobalConfig().Experimental.AllowsExpressionIndex
+	config.GetGlobalConfig().Experimental.AllowsExpressionIndex = true
+	defer func() {
+		config.GetGlobalConfig().Experimental.AllowsExpressionIndex = originalVal
+	}()
+
+	tk := testkit.NewTestKit(c, s.store)
+	tk.MustExec("use test")
+	tk.MustExec("drop table if exists t")
+	tk.MustExec("create table `t` (`a` int(11) default null, `b` int(11) default null, `c` int(11) default null, key `expression_index` ((case when `a` < 0 then 1 else 2 end)))")
+	_, err := tk.Exec("select * from t  where case when a < 0 then 1 else 2 end <= 1 order by 4;")
+	c.Assert(core.ErrUnknownColumn.Equal(err), IsTrue)
+}
+
+func (s *testIntegrationSuite) TestIssue26559(c *C) {
+	tk := testkit.NewTestKit(c, s.store)
+	tk.MustExec("use test")
+	tk.MustExec("create table t(a timestamp, b datetime);")
+	tk.MustExec("insert into t values('2020-07-29 09:07:01', '2020-07-27 16:57:36');")
+	tk.MustQuery("select greatest(a, b) from t union select null;").Sort().Check(testkit.Rows("2020-07-29 09:07:01", "<nil>"))
+}
+>>>>>>> 71638ee46... planner: the UNION's merge type should exclude the pure NULL (#26561)

planner/core/logical_plan_builder.go

@@ -1322,6 +1322,12 @@ func (b *PlanBuilder) buildDistinct(child LogicalPlan, length int) (*LogicalAggr
 // unionJoinFieldType finds the type which can carry the given types in Union.
 // Note that unionJoinFieldType doesn't handle charset and collation, caller need to handle it by itself.
 func unionJoinFieldType(a, b *types.FieldType) *types.FieldType {
+	// We ignore the pure NULL type.
+	if a.Tp == mysql.TypeNull {
+		return b
+	} else if b.Tp == mysql.TypeNull {
+		return a
+	}
 	resultTp := types.NewFieldType(types.MergeFieldType(a.Tp, b.Tp))
 	// This logic will be intelligible when it is associated with the buildProjection4Union logic.
 	if resultTp.Tp == mysql.TypeNewDecimal {