planner: fix incorrect TableDual plan built from nulleq

util/ranger/points.go

@@ -246,7 +246,8 @@ func (r *builder) buildFormBinOp(expr *expression.ScalarFunction) []*point {
 		if col.RetType.EvalType() == types.ETString && (value.Kind() == types.KindString || value.Kind() == types.KindBinaryLiteral) {
 			value.SetString(value.GetString(), col.RetType.Collate)
 		}
-		if col.GetType().Tp == mysql.TypeYear {
+		// If nulleq with null value, values.ToInt64 will return err
+		if col.GetType().Tp == mysql.TypeYear && !value.IsNull() {
 			// If the original value is adjusted, we need to change the condition.
 			// For example, col < 2156. Since the max year is 2155, 2156 is changed to 2155.
 			// col < 2155 is wrong. It should be col <= 2155.

util/ranger/ranger_test.go

@@ -1504,12 +1504,13 @@ func (s *testRangerSuite) TestIndexRangeForYear(c *C) {
 	// test index range
 	testKit.MustExec("DROP TABLE IF EXISTS t")
 	testKit.MustExec("CREATE TABLE t (a year(4), key(a))")
-	testKit.MustExec("INSERT INTO t VALUES (1), (70), (99), (0), ('0')")
+	testKit.MustExec("INSERT INTO t VALUES (1), (70), (99), (0), ('0'), (NULL)")
 	testKit.MustQuery("SELECT * FROM t WHERE a < 15698").Check(testkit.Rows("0", "1970", "1999", "2000", "2001"))
 	testKit.MustQuery("SELECT * FROM t WHERE a <= 0").Check(testkit.Rows("0"))
 	testKit.MustQuery("SELECT * FROM t WHERE a <= 1").Check(testkit.Rows("0", "1970", "1999", "2000", "2001"))
 	testKit.MustQuery("SELECT * FROM t WHERE a < 2000").Check(testkit.Rows("0", "1970", "1999"))
 	testKit.MustQuery("SELECT * FROM t WHERE a > -1").Check(testkit.Rows("0", "1970", "1999", "2000", "2001"))
+	testKit.MustQuery("SELECT * FROM t WHERE a <=> NULL").Check(testkit.Rows("<nil>"))
 
 	tests := []struct {
 		indexPos    int