planner: add docs for some variables

system-variables.md

@@ -335,6 +335,38 @@ This variable is an alias for `last_insert_id`.
 >
 > Unlike in MySQL, the `max_execution_time` system variable currently works on all kinds of statements in TiDB, not only restricted to the `SELECT` statement. The precision of the timeout value is roughly 100ms. This means the statement might not be terminated in accurate milliseconds as you specify.
 
+### max_prepared_stmt_count
+
+- Scope: GLOBAL
+- Persists to cluster: Yes
+- Type: Integer
+- Default value: `-1`
+- Range: `[-1, 1048576]`
+- Specifies the maximum number of [`PREPARE`](/sql-statements/sql-statement-prepare.md) statements in a session.
+- The value of `-1` means no limit on the maximum number of `PREPARE` statements in a session.
+- If you set the variable to a value that exceeds the upper limit `1048576`, `1048576` is used instead:
+
+```sql
+mysql> SET GLOBAL max_prepared_stmt_count=1048577;
+Query OK, 0 rows affected, 1 warning (0.01 sec)
+
+mysql> SHOW WARNINGS;
++---------+------+--------------------------------------------------------------+
+| Level   | Code | Message                                                      |
++---------+------+--------------------------------------------------------------+
+| Warning | 1292 | Truncated incorrect max_prepared_stmt_count value: '1048577' |
++---------+------+--------------------------------------------------------------+
+1 row in set (0.00 sec)
+
+mysql> SHOW GLOBAL VARIABLES LIKE 'max_prepared_stmt_count';
++-------------------------+---------+
+| Variable_name           | Value   |
++-------------------------+---------+
+| max_prepared_stmt_count | 1048576 |
++-------------------------+---------+
+1 row in set (0.00 sec)
+```
+
 ### plugin_dir
 
 - Scope: GLOBAL
@@ -1053,6 +1085,14 @@ Constraint checking is always performed in place for pessimistic transactions (d
 - Since v6.1.0, the [Join Reorder](/join-reorder.md) algorithm of TiDB supports Outer Join. This variable controls the support behavior, and the default value is `ON`.
 - For a cluster upgraded from a version earlier than v6.1.0, the default value is still `TRUE`.
 
+### tidb_enable_ordered_result_mode
+
+- Scope: SESSION | GLOBAL
+- Persists to cluster: Yes
+- Default value: `OFF`
+- Specifies whether to sort the final output result automatically.
+- For example, with this variable enabled, TiDB processes `SELECT a, MAX(b) FROM t GROUP BY a` as `SELECT a, MAX(b) FROM t GROUP BY a ORDER BY a, MAX(b)`.
+
 ### tidb_enable_paging <span class="version-mark">New in v5.4.0</span>
 
 - Scope: SESSION | GLOBAL
@@ -1724,6 +1764,35 @@ For a system upgraded to v5.0 from an earlier version, if you have not modified
 - This variable is used to set whether the optimizer executes the optimization operation of pushing down the aggregate function to the position before Join, Projection, and UnionAll.
 - When the aggregate operation is slow in query, you can set the variable value to ON.
 
+### tidb_opt_cartesian_bcj
+
+- Scope: SESSION | GLOBAL
+- Persists to cluster: YES
+- Type: Integer
+- Default value: `1`
+- Range: `[0, 2]`
+- Indicates whether to allow the Broadcast Cartesian Join. 
+- `0` means that the Broadcast Cartesian Join is not allowed. `1` means that it is allowed based on [`tidb_broadcast_join_threshold_count`](#tidb_broadcast_join_threshold_count-new-in-v50). `2` means that it is always allowed even if the table size exceeds the threshold.
+- This variable is internally used in TiDB, and it is not recommended to modify its value.
+
+### tidb_opt_concurrency_factor
+
+- Scope: SESSION | GLOBAL
+- Persists to cluster: YES
+- Type: Float
+- Range: `[0, 2147483647]`
+- Default value: `3.0`
+- Indicates the CPU cost of starting a Golang goroutine in TiDB. This variable is internally used in the [Cost Model](/cost-model.md), and it is not recommended to modify its value.
+
+### tidb_opt_cop_cpu_factor
+
+- Scope: SESSION | GLOBAL
+- Persists to cluster: YES
+- Type: Float
+- Range: `[0, 2147483647]`
+- Default value: `3.0`
+- Indicates the CPU cost for TiKV Coprocessor to process one row. This variable is internally used in the [Cost Model](/cost-model.md), and it is not recommended to modify its value.
+
 ### tidb_opt_correlation_exp_factor
 
 - Scope: SESSION | GLOBAL
@@ -1746,6 +1815,33 @@ For a system upgraded to v5.0 from an earlier version, if you have not modified
 - Range: `[0, 1]`
 - This variable is used to set the threshold value that determines whether to enable estimating the row count by using column order correlation. If the order correlation between the current column and the `handle` column exceeds the threshold value, this method is enabled.
 
+### tidb_opt_cpu_factor
+
+- Scope: SESSION | GLOBAL
+- Persists to cluster: YES
+- Type: Float
+- Range: `[0, 2147483647]`
+- Default value: `3.0`
+- Indicates the CPU cost for TiDB to process one row. This variable is internally used in the [Cost Model](/cost-model.md), and it is not recommended to modify its value.
+
+### tidb_opt_desc_scan_factor
+
+- Scope: SESSION | GLOBAL
+- Persists to cluster: YES
+- Type: Float
+- Range: `[0, 2147483647]`
+- Default value: `3.0`
+- Indicates the cost for TiKV to scan one row from the disk in descending order. This variable is internally used in the [Cost Model](/cost-model.md), and it is not recommended to modify its value.
+
+### tidb_opt_disk_factor
+
+- Scope: SESSION | GLOBAL
+- Persists to cluster: YES
+- Type: Float
+- Range: `[0, 2147483647]`
+- Default value: `1.5`
+- Indicates the I/O cost for TiDB to read or write one byte of data from or to the temporary disk. This variable is internally used in the [Cost Model](/cost-model.md), and it is not recommended to modify its value.
+
 ### tidb_opt_distinct_agg_push_down
 
 - Scope: SESSION
@@ -1825,6 +1921,15 @@ mysql> desc select count(distinct a) from test.t;
 - This variable is used to set the threshold that determines whether to push the Limit or TopN operator down to TiKV.
 - If the value of the Limit or TopN operator is smaller than or equal to this threshold, these operators are forcibly pushed down to TiKV. This variable resolves the issue that the Limit or TopN operator cannot be pushed down to TiKV partly due to wrong estimation.
 
+### tidb_opt_memory_factor
+
+- Scope: SESSION | GLOBAL
+- Persists to cluster: YES
+- Type: Float
+- Range: `[0, 2147483647]`
+- Default value: `0.001`
+- Indicates the memory cost for TiDB to store one row. This variable is internally used in the [Cost Model](/cost-model.md), and it is not recommended to modify its value.
+
 ### tidb_opt_mpp_outer_join_fixed_build_side <span class="version-mark">New in v5.1.0</span>
 
 - Scope: SESSION | GLOBAL
@@ -1833,6 +1938,15 @@ mysql> desc select count(distinct a) from test.t;
 - Default value: `ON`
 - Specifies whether to allow using outer table as the build side in outer join for TiFlash.
 
+### tidb_opt_network_factor
+
+- Scope: SESSION | GLOBAL
+- Persists to cluster: YES
+- Type: Float
+- Range: `[0, 2147483647]`
+- Default value: `1.0`
+- Indicates the net cost of transferring 1 byte of data through the network. This variable is internally used in the [Cost Model](/cost-model.md), and it is not recommended to modify its value.
+
 ### tidb_opt_prefer_range_scan <span class="version-mark">New in v5.0</span>
 
 - Scope: SESSION | GLOBAL
@@ -1874,6 +1988,24 @@ explain select * from t where age=5;
 - Default value: `OFF`
 - Specifies whether to allow the optimizer to push `Projection` down to the TiKV or TiFlash coprocessor.
 
+### tidb_opt_scan_factor
+
+- Scope: SESSION | GLOBAL
+- Persists to cluster: YES
+- Type: Float
+- Range: `[0, 2147483647]`
+- Default value: `1.5`
+- Indicates the cost for TiKV to scan one row of data from the disk in ascending order. This variable is internally used in the [Cost Model](/cost-model.md), and it is not recommended to modify its value.
+
+### tidb_opt_seek_factor
+
+- Scope: SESSION | GLOBAL
+- Persists to cluster: YES
+- Type: Float
+- Range: `[0, 2147483647]`
+- Default value: `20`
+- Indicates the start-up cost for TiDB to request data from TiKV. This variable is internally used in the [Cost Model](/cost-model.md), and it is not recommended to modify its value.
+
 ### tidb_opt_skew_distinct_agg <span class="version-mark">New in v6.2.0</span>
 
 > **Note:**