set tidb_enable_global_index = true;
mysql> show create table tpk2;
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                                                                                                                                                                                                                                             |
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| tpk2  | CREATE TABLE `tpk2` (
  `a` int(11) DEFAULT NULL,
  `b` int(11) DEFAULT NULL,
  `c` int(11) NOT NULL,
  `d` int(11) NOT NULL AUTO_INCREMENT,
  KEY `idx_bc` (`b`,`c`),
  UNIQUE KEY `uidx_ac` (`a`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin AUTO_INCREMENT=30002
PARTITION BY HASH (`c`) PARTITIONS 2 |
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

insert into tpk2 values (1, 1, 1, 1), (2, 2, 2, 2);

mysql> select /*+ use_index_merge(tpk2, uidx_ac, idx_bc) */ * from tpk2 where a=1 or b=2;
+------+------+---+---+
| a    | b    | c | d |
+------+------+---+---+
|    2 |    2 | 2 | 2 |
+------+------+---+---+
1 row in set (0.01 sec)

mysql> select * from tpk2 where a=1 or b=2;
+------+------+---+---+
| a    | b    | c | d |
+------+------+---+---+
|    2 |    2 | 2 | 2 |
|    1 |    1 | 1 | 1 |
+------+------+---+---+
2 rows in set (0.01 sec)