txn: update the document about read-consistency read check ts variable (

#7928)

system-variables.md

@@ -1452,6 +1452,18 @@ Usage example:
 SET tidb_query_log_max_len = 20
 ```
 
+### tidb_rc_read_check_ts (New in v6.0.0)
+
+> **Warning:**
+>
+> - This feature is incompatible with [`replica-read`](#tidb_replica_read-new-in-v40). Do not enable `tidb_rc_read_check_ts` and `replica-read` at the same time.
+> - If your client uses a cursor, it is not recommended to enable `tidb_rc_read_check_ts` in case that the previous batch of returned data has already been used by the client and the statement eventually fails.
+
+- Scope: SESSION | GLOBAL
+- Default value: `OFF`
+- This variable is used to optimize the timestamp acquisition, which is suitable for scenarios with read-committed isolation level where read-write conflicts are rare. Enabling this variable can avoid the latency and cost of getting the global timestamp, and can optimize the transaction-level read latency.
+- If read-write conflicts are severe, enabling this feature will increase the cost and latency of getting the global timestamp, and might cause performance regression. For details, see [Read Committed isolation level](/transaction-isolation-levels.md#read-committed-isolation-level).
+
 ### tidb_read_staleness <span class="version-mark">New in v5.4.0</span>
 
 - Scope: SESSION

transaction-isolation-levels.md

@@ -62,6 +62,15 @@ For historical reasons, the Read Committed isolation level of current mainstream
 >
 > The Read Committed isolation level only takes effect in the [pessimistic transaction mode](/pessimistic-transaction.md). In the [optimistic transaction mode](/optimistic-transaction.md), setting the transaction isolation level to `Read Committed` does not take effect and transactions still use the Repeatable Read isolation level.
 
+Starting from v6.0.0, TiDB supports using the [`tidb_rc_read_check_ts`](/system-variables.md#tidb_rc_read_check_ts-new-in-v600) system variable to optimize the timestamp acquisition in scenarios where read-write conflicts are rare. After enabling this variable, TiDB will try to use the previous valid timestamp to read data when `SELECT` is executed. The initial value of this variable is the `start_ts` of the transaction.
+
+- If TiDB does not encounter any data update during the read process, it returns the result to the client and the `SELECT` statement is successfully executed.
+- If TiDB encounters data update during the read process:
+    - If TiDB has not yet sent the result to the client, TiDB tries to acquire a new timestamp and retry this statement.
+    - If TiDB has already sent partial data to the client, TiDB reports an error to the client. The amount of data sent to the client each time is controlled by `tidb_init_chunk_size` and `tidb_max_chunk_size`.
+
+In scenarios where the `READ-COMMITTED` isolation level is used, the `SELECT` statements are many, and read-write conflicts are rare, enabling this variable can avoid the latency and cost of getting the global timestamp.
+
 ## Difference between TiDB and MySQL Read Committed
 
 The MySQL Read Committed isolation level is in line with the Consistent Read features in most cases. There are also exceptions, such as [semi-consistent read](https://dev.mysql.com/doc/refman/8.0/en/innodb-transaction-isolation-levels.html). This special behavior is not supported in TiDB.