system variable: add tidb_lock_unchanged_keys

Signed-off-by: Aolin <[email protected]>

system-variables.md

@@ -2797,6 +2797,17 @@ For a system upgraded to v5.0 from an earlier version, if you have not modified
 
 </CustomContent>
 
+### `tidb_lock_unchanged_keys` <span class="version-mark">New in v7.1.1</span>
+
+- Scope: SESSION | GLOBAL
+- Persists to cluster: Yes
+- Type: Boolean
+- Default value: `ON`
+- This variable is used to control whether to lock specific keys in the following scenarios. When the value is set to `ON`, these keys are locked. When the value is set to `OFF`, no keys are locked.
+    - Duplicate keys in `INSERT IGNORE` and `REPLACE` statements. Before v6.1.6, these keys were not locked. This issue has been fixed in [#42121](https://github.com/pingcap/tidb/issues/42121).
+    - Unique keys in `UPDATE` statements when the values of the keys are not changed. Before v6.5.2, these keys were not locked. This issue has been fixed in [#36438](https://github.com/pingcap/tidb/issues/36438).
+- To maintain the consistency and rationality of the transaction, it is not recommended to change this value. If upgrading TiDB causes severe performance issues due to these two fixes, and the unlocked behavior is acceptable (see the preceding issues), you can set this variable to `OFF`.
+
 ### tidb_log_file_max_days <span class="version-mark">New in v5.3.0</span>
 
 - Scope: GLOBAL