add docs for enforce mpp

system-variables.md

@@ -206,8 +206,11 @@ mysql> SELECT * FROM t1;
 
 - Scope: SESSION | GLOBAL
 - Default value: `ON`
-- Possible values: `OFF`, `ON`, `ENFORCE`
-- This variable controls whether to use the MPP mode of TiFlash to execute queries. If the value is set to `ON`, TiDB automatically determines using the optimizer whether to choose MPP to execute queries. 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.
+- 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.
+    - `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.md#control-whether-to-select-the-mpp-mode).
 
 ### tidb_allow_remove_auto_inc <span class="version-mark">New in v2.1.18 and v3.0.4</span>
 
@@ -606,6 +609,16 @@ Query OK, 0 rows affected (0.09 sec)
 - Default value: `ON`
 - This variable is used to control whether to enable the support for window functions. Note that window functions may use reserved keywords. This might cause SQL statements that could be executed normally cannot be parsed after upgrading TiDB. In this case, you can set `tidb_enable_window_function` to `OFF`.
 
+### `tidb_enforce_mpp` <span class="version-mark">New in v5.1</span>
+
+- Scope: SESSION
+- Default value: `OFF`
+- Controls whether to ignore the optimizer's cost estimation and to forcibly use TiFlash's MPP mode for query execution. The value options are as follows:
+    - `0` or `OFF`, which means that the MPP mode is not forcibly used (by default).
+    - `1` or `ON`, which means that the cost estimation is ignored and the MPP mode is forcibly used. Note that this setting only takes effect when `tidb_allow_mpp=true`.
+
+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.md#control-whether-to-select-the-mpp-mode).
+
 ### tidb_evolve_plan_baselines <span class="version-mark">New in v4.0</span>
 
 - Scope: SESSION | GLOBAL

tidb-configuration-file.md

@@ -439,6 +439,12 @@ Configuration items related to performance.
 + Default value: `20971520`
 + When `nested-loop-join-cache-capacity` is set to `0`, nested loop join cache is disabled by default. When the LRU size is larger than the value of `nested-loop-join-cache-capacity`, the elements in the LRU are removed.
 
+### `enforce-mpp`
+
++ Determines whether to ignore the optimizer's cost estimation and to forcibly use TiFlash's MPP mode for query execution.
++ Default value: `false`
++ This configuration item is the initial value of [`tidb_enforce_mpp`](/system-variables.md#tidb_enforce_mpp-new-in-v51).
+
 ## prepared-plan-cache
 
 The Plan Cache configuration of the `PREPARE` statement.

tiflash/use-tiflash.md

@@ -193,7 +193,7 @@ In the above three ways of reading TiFlash replicas, engine isolation specifies
 
 > **Note:**
 >
-> Before v4.0.3, the behavior of reading from TiFlash replica in a non-read-only SQL statement (for example, `INSERT INTO ... SELECT`, `SELECT ... FOR UPDATE`, `UPDATE ...`, `DELETE ...`) is undefined. In v4.0.3 and later versions, internally TiDB ignores the TiFlash replica for a non-read-only SQL statement to guarantee the data correctness. That is, for [smart selection](#smart-selection), TiDB automatically chooses the non-TiFlash replica; for [engine isolation](#engine-isolation) that specifies TiFlash replica **only**, TiDB reports an error; and for [manual hint](#manual-hint), TiDB ignores the hint.
+> Before v4.0.3, the behavior of reading from TiFlash replica in a non-read-only SQL statement (for example, `INSERT INTO ... SELECT`, `SELECT ... FOR UPDATE`, `UPDATE ...`, `DELETE ...`) is undefined. In v4.0.3 and later versions, internally TiDB ignores the TiFlash replica for a non-read-only SQL statement to guarantee the data correctness. That is, for [smart selection](#smart-selection), TiDB automatically selects the non-TiFlash replica; for [engine isolation](#engine-isolation) that specifies TiFlash replica **only**, TiDB reports an error; and for [manual hint](#manual-hint), TiDB ignores the hint.
 
 ## Use TiSpark to read TiFlash replicas
 
@@ -259,13 +259,74 @@ If a query encounters unsupported push-down calculations, TiDB needs to complete
 
 ## Use the MPP mode
 
-TiFlash supports using the MPP mode to execute queries, which introduces cross-node data exchange (data shuffle process) into the computation. The MPP mode is enabled by default and can be disabled by setting the value of the global/session variable [`tidb_allow_mpp`](/system-variables.md#tidb_allow_mpp-new-in-v50) to `0` or `OFF`.
+TiFlash supports using the MPP mode to execute queries, which introduces cross-node data exchange (data shuffle process) into the computation. TiDB automatically determines whether to select the MPP mode using the optimizer's cost estimation. You can change the selection strategy by modifying the values of [`tidb_allow_mpp`](/system-variables.md#tidb_allow_mpp-new-in-v50) and [`tidb_enforce_mpp`](/system-variables.md#tidb_enforce_mpp-new-in-v51).
 
-```shell
-set @@session.tidb_allow_mpp=0
+### Control whether to select the MPP mode
+
+The `tidb_allow_mpp` variable controls whether TiDB can select the MPP mode to execute queries. The `tidb_enforce_mpp` variable controls whether the optimizer's cost estimation is ignored and the MPP mode of TiFlash is forcibly used to execute queries.
+
+The results corresponding to all values of these two variables are as follows:
+
+|                        | tidb_allow_mpp=off | tidb_allow_mpp=on (by default)              |
+| ---------------------- | -------------------- | -------------------------------- |
+| tidb_enforce_mpp=off (by default) | The MPP mode is not used. | The optimizer selects the MPP mode based on cost estimation. (by default)|
+| tidb_enforce_mpp=on  | The MPP mode is not used.   | TiDB ignores the cost estimation and selects the MPP mode.      |
+
+For example, if you do not want to use the MPP mode, you can execute the following statements:
+
+{{< copyable "sql" >}}
+
+```sql
+set @@session.tidb_allow_mpp=1;
+set @@session.tidb_enforce_mpp=0;
 ```
 
-MPP mode supports these physical algorithms: Broadcast Hash Join, Shuffled Hash Join, Shuffled Hash Aggregation, Union All, TopN, and Limit. The optimizer automatically determines which algorithm to be used in a query. To check the specific query execution plan, you can execute the `EXPLAIN` statement. If the result of the `EXPLAIN` statement shows ExchangeSender and ExchangeReceiver operators, it indicates that the MPP mode has taken effect.
+If you want TiDB's cost-based optimizer to automatically decide whether to use the MPP mode (by default), you can execute the following statements:
+
+{{< copyable "sql" >}}
+
+```sql
+set @@session.tidb_allow_mpp=1;
+set @@session.tidb_enforce_mpp=0;
+```
+
+If you want TiDB to ignore the optimizer's cost estimation and to forcibly select the MPP mode, you can execute the following statements:
+
+{{< copyable "sql" >}}
+
+```sql
+set @@session.tidb_allow_mpp=1;
+set @@session.tidb_enforce_mpp=1;
+```
+
+The initial value of the `tidb_enforce_mpp` session variable is equal to the [`enforce-mpp`](/tidb-configuration-file.md#enforce-mpp) configuration value of this tidb-server instance (which is `false` by default). If multiple tidb-server instances in a TiDB cluster only perform analytical queries and you want to make sure that the MPP mode is used on these instances, you can change their [`enforce-mpp`](/tidb-configuration-file.md#enforce-mpp) configuration values to `true`.
+
+> **Note:**
+>
+> When `tidb_enforce_mpp=1` takes effect, the TiDB optimizer will ignore the cost estimation to choose the MPP mode. However, if other factors block the MPP mode, TiDB will not select the MPP mode. These factors include the absence of TiFlash replica, unfinished replication of TiFlash replicas, and statements containing operators or functions that are not supported by the MPP mode.
+> 
+> If TiDB optimizer cannot select the MPP mode due to reasons other than cost estimation, when you use the `EXPLAIN` statement to check out the execution plan, a warning is returned to explain the reason. For example:
+> 
+> {{< copyable "sql" >}}
+> 
+> ```sql
+> set @@session.tidb_enforce_mpp=1;
+> create table t(a int);
+> explain select count(*) from t; 
+> show warnings;
+> ```
+> 
+> ```
+> +---------+------+-----------------------------------------------------------------------------+
+> | Level   | Code | Message                                                                     |
+> +---------+------+-----------------------------------------------------------------------------+
+> | Warning | 1105 | MPP mode may be blocked because there aren't tiflash replicas of table `t`. |
+> +---------+------+-----------------------------------------------------------------------------+
+> ```
+
+### Algorithm support for the MPP mode
+
+The MPP mode supports these physical algorithms: Broadcast Hash Join, Shuffled Hash Join, Shuffled Hash Aggregation, Union All, TopN, and Limit. The optimizer automatically determines which algorithm to be used in a query. To check the specific query execution plan, you can execute the `EXPLAIN` statement. If the result of the `EXPLAIN` statement shows ExchangeSender and ExchangeReceiver operators, it indicates that the MPP mode has taken effect.
 
 The following statement takes the table structure in the TPC-H test set as an example:
 
@@ -330,4 +391,4 @@ Currently, TiFlash does not support some features. These features might be incom
         In the example above, `a/b`'s inferred type from the compiling is `Decimal(7,4)` both in TiDB and in TiFlash. Constrained by `Decimal(7,4)`, `a/b`'s returned type should be `0.0000`. In TiDB, `a/b`'s runtime precision is higher than `Decimal(7,4)`, so the original table data is not filtered by the `where a/b` condition. However, in TiFlash, the calculation of `a/b` uses `Decimal(7,4)` as the result type, so the original table data is filtered by the `where a/b` condition.
 
 * The MPP mode in TiFlash does not support the following features:
-    * If the [`new_collations_enabled_on_first_bootstrap`](/tidb-configuration-file.md#new_collations_enabled_on_first_bootstrap) configuration item's value is `true`, the MPP mode does not support the string-type join key or the string column type in the `group by` aggregation. For these two query types, the MPP mode is not chosen by default.
+    * If the [`new_collations_enabled_on_first_bootstrap`](/tidb-configuration-file.md#new_collations_enabled_on_first_bootstrap) configuration item's value is `true`, the MPP mode does not support the string-type join key or the string column type in the `group by` aggregation. For these two query types, the MPP mode is not selected by default.