tiflash: Add note about mpp plan change (#17428)

releases/release-7.3.0.md

@@ -155,6 +155,10 @@ Quick access: [Quick start](https://docs.pingcap.com/tidb/v7.3/quick-start-with-
 
 ### Behavior changes
 
+* TiDB
+
+    - MPP is a distributed computing framework provided by the TiFlash engine, which allows data exchange between nodes and provides high-performance, high-throughput SQL algorithms. Compared with other protocols, the MPP protocol is more mature and can provide better task and resource management. Starting from v7.3.0, when TiDB pushes computation tasks to TiFlash, the optimizer only generates execution plans using the MPP protocol by default. If [`tidb_allow_mpp`](/system-variables.md#tidb_allow_mpp-new-in-v50) is set to `OFF`, queries might return errors after you upgrade TiDB. It is recommended that you check the value of `tidb_allow_mpp` and set it to `ON` before the upgrade. If you still need the optimizer to choose one of the Cop, BatchCop, and MPP protocols for generating execution plans based on cost estimates, you can set the [`tidb_allow_tiflash_cop`](/system-variables.md#tidb_allow_tiflash_cop-new-in-v730) variable to `ON`.
+
 * Backup & Restore (BR)
 
     - BR adds an empty cluster check before performing a full data restoration. By default, restoring data to a non-empty cluster is not allowed. If you want to force the restoration, you can use the `--filter` option to specify the corresponding table name to restore data to.

system-variables.md

@@ -1089,7 +1089,7 @@ mysql> SHOW GLOBAL VARIABLES LIKE 'max_prepared_stmt_count';
 - Type: Boolean
 - Default value: `ON`
 - Controls whether to use the MPP mode of TiFlash to execute queries. The value options are as follows:
-    - `0` or `OFF`, which means that the MPP mode will not be used.
+    - `0` or `OFF`, which means that the MPP mode will not be used. For v7.3.0 or a later version, if you set the value of this variable to `0` or `OFF`, you also need to enable the [`tidb_allow_tiflash_cop`](/system-variables.md#tidb_allow_tiflash_cop-new-in-v730) variable. Otherwise, queries might return errors.
     - `1` or `ON`, which means that the optimizer determines whether to use the MPP mode based on the cost estimation (by default).
 
 MPP is a distributed computing framework provided by the TiFlash engine, which allows data exchange between nodes and provides high-performance, high-throughput SQL algorithms. For details about the selection of the MPP mode, refer to [Control whether to select the MPP mode](/tiflash/use-tiflash-mpp-mode.md#control-whether-to-select-the-mpp-mode).