planner: Add optimizer risk variables

system-variables.md

@@ -4625,6 +4625,32 @@ SHOW WARNINGS;
 
 </details>
 
+### tidb_opt_risk_eq_skew_ratio <span class="version-mark">New in v9.0.0</span>
+
+> **Warning:**
+>
+> The TiDB query optimizer uses this variable. Do not adjust this variable without guidance from TiDB support or engineering.
+
+- Scope: SESSION | GLOBAL
+- Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): Yes
+- Type: Float
+- Range: `[0, 1]`
+- Default value: `0`
+- This variable controls the ratio of risk between the original predicate selectivity estimate and the potential skew estimate for an equality predicate, when the value is not found in TopN or histogram buckets.
+
+### tidb_opt_risk_range_skew_ratio <span class="version-mark">New in v9.0.0</span>
+
+> **Warning:**
+>
+> The TiDB query optimizer uses this variable. Do not adjust this variable without guidance from TiDB support or engineering.
+
+- Scope: SESSION | GLOBAL
+- Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): Yes
+- Type: Float
+- Range: `[0, 1]`
+- Default value: `0`
+- This variable controls the ratio of risk between the original predicate selectivity estimate and the potential skew estimate for a range predicate, when the search range is found within a single histogram bucket or entirely outside the range of all histogram buckets.
+
 ### tidb_opt_scan_factor
 
 - Scope: SESSION | GLOBAL