参考文档：
https://docs.pingcap.com/zh/tidb/stable/sql-statement-split-region#split-table-region
均匀切分
由于 row_id 是整数，所以根据指定的 lower_value、upper_value 以及 region_num，可以推算出需要切分的 key。TiDB 先计算 step (step = (upper_value - lower_value)/region_num)，然后在 lower_value 和 upper_value 之间每隔 step 区间切一次，最终切出 region_num 个 Region。
例如，对于表 t，如果想要从 minInt64~maxInt64 之间均匀切割出 16 个 Region，可以用以下语句：
SPLIT TABLE t BETWEEN (-9223372036854775808) AND (9223372036854775807) REGIONS 16;
该语句会把表 t 从 minInt64 到 maxInt64 之间均匀切割出 16 个 Region。如果已知主键的范围没有这么大，比如只会在 0~1000000000 之间，那可以用 0 和 1000000000 分别代替上面的 minInt64 和 maxInt64 来切分 Region。
SPLIT TABLE t BETWEEN (0) AND (1000000000) REGIONS 16;
不均匀切分
如果已知数据不是均匀分布的，比如想要 -inf ~ 10000 切一个 Region，10000 ~ 90000 切一个 Region，90000 ~ +inf 切一个 Region，可以通过手动指定点来切分 Region，示例如下：
SPLIT TABLE t BY (10000), (90000);
大家一直在强调varchar，我还以为varchar不能拆分呢。特意测试了下，可以的。
root@127.0.0.1:4000[hqh]>show create table t1;
+-------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table |
+-------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| t1 | CREATE TABLE `t1` (
`id` varchar(64) NOT NULL,
`intkey` int(11) NOT NULL,
`pad1` varbinary(1024) DEFAULT NULL,
KEY `intkey` (`intkey`),
PRIMARY KEY (`id`) /*T![clustered_index] NONCLUSTERED */
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin |
+-------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.01 sec)
root@127.0.0.1:4000[hqh]>
root@127.0.0.1:4000[hqh]>show table t1 regions;
+-----------+-----------+---------+-----------+-----------------+-------+------------+---------------+------------+----------------------+------------------+
| REGION_ID | START_KEY | END_KEY | LEADER_ID | LEADER_STORE_ID | PEERS | SCATTERING | WRITTEN_BYTES | READ_BYTES | APPROXIMATE_SIZE(MB) | APPROXIMATE_KEYS |
+-----------+-----------+---------+-----------+-----------------+-------+------------+---------------+------------+----------------------+------------------+
| 1003 | t_110_ | | 1004 | 1 | 1004 | 0 | 2700 | 1355 | 1 | 0 |
+-----------+-----------+---------+-----------+-----------------+-------+------------+---------------+------------+----------------------+------------------+
1 row in set (0.03 sec)
root@127.0.0.1:4000[hqh]>
root@127.0.0.1:4000[hqh]>SPLIT TABLE t1 BY (10000), (90000);
+--------------------+----------------------+
| TOTAL_SPLIT_REGION | SCATTER_FINISH_RATIO |
+--------------------+----------------------+
| 2 | 1 |
+--------------------+----------------------+
1 row in set (0.01 sec)
root@127.0.0.1:4000[hqh]>
root@127.0.0.1:4000[hqh]>show table t1 regions;
+-----------+---------------+---------------+-----------+-----------------+--------+------------+---------------+------------+----------------------+------------------+
| REGION_ID | START_KEY | END_KEY | LEADER_ID | LEADER_STORE_ID | PEERS | SCATTERING | WRITTEN_BYTES | READ_BYTES | APPROXIMATE_SIZE(MB) | APPROXIMATE_KEYS |
+-----------+---------------+---------------+-----------+-----------------+--------+------------+---------------+------------+----------------------+------------------+
| 108003 | t_110_ | t_110_r_10000 | 108004 | 1 | 108004 | 0 | 0 | 0 | 1 | 0 |
| 108005 | t_110_r_10000 | t_110_r_90000 | 108006 | 1 | 108006 | 0 | 0 | 0 | 1 | 0 |
| 1003 | t_110_r_90000 | | 1004 | 1 | 1004 | 0 | 0 | 0 | 1 | 0 |
+-----------+---------------+---------------+-----------+-----------------+--------+------------+---------------+------------+----------------------+------------------+
3 rows in set (0.01 sec)