Update system-variables tidb_ddl_reorg_batch_size (#18616)

system-variables.md

@@ -1694,8 +1694,8 @@ mysql> SELECT job_info FROM mysql.analyze_jobs ORDER BY end_time DESC LIMIT 1;
 - Range: `[32, 10240]`
 - Unit: Rows
 - This variable is used to set the batch size during the `re-organize` phase of the DDL operation. For example, when TiDB executes the `ADD INDEX` operation, the index data needs to backfilled by `tidb_ddl_reorg_worker_cnt` (the number) concurrent workers. Each worker backfills the index data in batches.
-    - If many updating operations such as `UPDATE` and `REPLACE` exist during the `ADD INDEX` operation, a larger batch size indicates a larger probability of transaction conflicts. In this case, you need to adjust the batch size to a smaller value. The minimum value is 32.
-    - If the transaction conflict does not exist, you can set the batch size to a large value (consider the worker count. See [Interaction Test on Online Workloads and `ADD INDEX` Operations](https://docs.pingcap.com/tidb/stable/online-workloads-and-add-index-operations) for reference). This can increase the speed of the backfilling data, but the write pressure on TiKV also becomes higher.
+    - If `tidb_ddl_enable_fast_reorg` is set to `OFF`, `ADD INDEX` is executed as a transaction. If there are many update operations such as `UPDATE` and `REPLACE` in the target columns during the `ADD INDEX` execution, a larger batch size indicates a larger probability of transaction conflicts. In this case, it is recommended that you set the batch size to a smaller value. The minimum value is 32.
+    - If the transaction conflict does not exist, or if `tidb_ddl_enable_fast_reorg` is set to `ON`, you can set the batch size to a large value. This makes data backfilling faster but also increases the write pressure on TiKV. For a proper batch size, you also need to refer to the value of `tidb_ddl_reorg_worker_cnt`. See [Interaction Test on Online Workloads and `ADD INDEX` Operations](https://docs.pingcap.com/tidb/dev/online-workloads-and-add-index-operations) for reference.
 
 ### tidb_ddl_reorg_priority