planner: prepared plan cache support limit

sql-prepared-plan-cache.md

@@ -23,7 +23,7 @@ In the current version of TiDB, if a `Prepare` statement meets any of the follow
 - The query contains the `ignore_plan_cache` hint, such as `select /*+ ignore_plan_cache() */ * from t`.
 - The query contains variables other than `?` (including system variables or user-defined variables), such as `select * from t where a>? and b>@x`.
 - The query contains the functions that cannot be cached: `database()`, `current_user`, `current_role`, `user`, `connection_id`, `last_insert_id`, `row_count`, `version`, and `like`.
-- The query contains `?` after `Limit`, such as `Limit ?` and `Limit 10, ?`. Such queries are not cached because the specific value of `?` has a great impact on query performance.
+- The query with `Count` being `?` after `Limit` and the `Count` value is greater than 10000.
 - The query contains `?` after `Order By`, such as `Order By ?`. Such queries sort data based on the column specified by `?`. If the queries targeting different columns use the same execution plan, the results will be wrong. Therefore, such queries are not cached. However, if the query is a common one, such as `Order By a+?`, it is cached.
 - The query contains `?` after `Group By`, such as `Group By?`. Such queries group data based on the column specified by `?`. If the queries targeting different columns use the same execution plan, the results will be wrong. Therefore, such queries are not cached. However, if the query is a common one, such as `Group By a+?`, it is cached.
 - The query contains `?` in the definition of the `Window Frame` window function, such as `(partition by year order by sale rows ? preceding)`. If `?` appears elsewhere in the window function, the query is cached.

system-variables.md

@@ -1726,7 +1726,15 @@ MPP is a distributed computing framework provided by the TiFlash engine, which a
 - Persists to cluster: Yes
 - Type: Boolean
 - Default value: `ON`
-- This variables specifies whether to use the pipeline execution algorithm for window functions.
+- This variable specifies whether to use the pipeline execution algorithm for window functions.
+
+### `tidb_enable_plan_cache_for_param_limit` <span class="version-mark">New in v6.6.0</span>
+
+- Scope: SESSION | GLOBAL
+- Persists to cluster: Yes
+- Type: Boolean
+- Default value: `ON`
+- This variable controls whether Prepared Plan Cache caches execution plans with `Count` being `?` after `Limit`. The default value is `ON`, which means Prepared Plan Cache supports caching such execution plans. Note that Prepared Plan Cache does not support caching execution plans with a `Count` that is greater than 10000.
 
 ### tidb_enable_plan_replayer_capture