The table has the Composite Primary Key will be very slow while using Dumpling to backup in MySQL

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Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

I found that logical backup is very slow while using Dumping in MySQL.

1. The huge table has a Composite Primary Key and the first column which isn't the type of integer.
2. With -r option while using dumpling
3. Table schema like :

CREATE TABLE `f_sn` (
  `f_id` varchar(31) NOT NULL,
  `version` int(11) NOT NULL,
  `revision` int(11) NOT NULL,
  `h_id` bigint(20) NOT NULL DEFAULT '0',
  `v_type` int(4) NOT NULL DEFAULT '0',
  PRIMARY KEY (`f_id`,`version`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8

2. What did you expect to see? (Required)

While meeting the column which is not an integer in the first column in Composite Primary Key just like Mydumper. Don't use the integer column to split chunks.

3. What did you see instead (Required)

Dumpling will use the integer column to split chunks.
Just like :

-- session 1
select * from f_sn where `version` >= 1000 and `version` < 2000 order by f_id,version;
-- session 2
select * from f_sn where `version` >= 2000 and `version` < 3000 order by f_id,version;

4. What is your TiDB version? (Required)

Mysql: 5.7.27
Dumpling : v4.0.10 and v5.2.0

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