planner: the UNION's merge type should exclude the pure NULL

planner/core/integration_test.go

@@ -3996,3 +3996,11 @@ func (s *testIntegrationSerialSuite) TestIssue26214(c *C) {
 	_, 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>"))
+}

planner/core/logical_plan_builder.go

@@ -1324,6 +1324,12 @@ 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 {