add tidb_optimizer_selectivity_level

Author: TomShawn

system-variables.md

@@ -2520,6 +2520,14 @@ explain select * from t where age=5;
 - Default value: `OFF`
 - This variable is used to control whether common table expressions (CTEs) in the entire session are inlined or not. The default value is `OFF`, which means that inlining CTE is not enforced by default. However, you can still inline CTE by specifying the `MERGE()` hint. If the variable is set to `ON`, all CTEs (except recursive CTE) in this session are forced to be inlined.
 
+### tidb_optimizer_selectivity_level
+
+- Scope: SESSION | GLOBAL
+- Persists to cluster: Yes
+- Default value: `1`
+- Value options: `1` and `2` (not recommended)
+- This variable controls the iteration of the optimizer's estimation logic. After changing the value of this variable, the estimation logic of the optimizer will change greatly. Currently, `1` is the only valid value. It is not recommended to set the value to `2`.
+
 ### tidb_partition_prune_mode <span class="version-mark">New in v5.1</span>
 
 - Scope: SESSION | GLOBAL