global sort ADD INDEX can adjust performance with tidb_ddl_reorg_worker_cnt

[lance6716]

No description provided.

[lance6716]

closed by #51261

below are global sort metrics

worker_cnt = 4

worker_cnt = 16

 CREATE TABLE `item` (
...
  `a` varchar(127) DEFAULT NULL,
  `b` json DEFAULT NULL,
...
  KEY `worker_cnt_4` ((cast(`b` as char(1000) array)),`a`),
  KEY `worker_cnt_16` ((cast(`b` as char(1000) array)),`a`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin

cc @tangenta