Table with range columns can't add partition

[fatpa]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?

Table Schema is

CREATE TABLE `test4` (
  `stime` datetime DEFAULT NULL,
  KEY `stime` (`stime`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin 
PARTITION BY RANGE columns (stime) (
  PARTITION p20190901 VALUES LESS THAN ('2019-09-02 00:00:00'),
  PARTITION p20190902 VALUES LESS THAN ('2019-09-03 00:00:00')
);

and run the sql

ALTER TABLE test4 add partition (PARTITION p20190903 VALUES LESS THAN ('2019-09-04 00:00:00'));

2. What did you expect to see?
   Query OK, 0 rows affected

3. What did you see instead?
   ERROR 1659 (HY000): Field '"2019-09-02 00:00:00"' is of a not allowed type for this type of partitioning

4. What version of TiDB are you using (tidb-server -V or run select tidb_version(); on TiDB)?
   V3.0.2

[fatpa]

It seems that can't work with varchar or datetime type.

[tiancaiamao]

Well, partition by range and partition by range columns shares some code, and partition by range has the constraint that only int value is allowed in the partition expression, this constraint is mistakenly imposed on partition by range columns and cause this bug.

[fatpa]

The TODO onddl/partition.go

// TODO: Remove the test 's.Partition.Expr == nil' when we support 'PARTITION BY RANGE COLUMNS'
if len(constraints) == 0 || s.Partition.Expr == nil {
	return nil
}

It looks like that the condition s.Partition.Expr == nil needs to be removed.

[tiancaiamao]

@crazycs520 is working on it.

[fatpa]

Duplicate of #12792