tidb: add the join order related variable tidb_opt_join_reorder_through_sel

system-variables.md

@@ -4545,6 +4545,17 @@ mysql> desc select count(distinct a) from test.t;
 - This variable is used to control the selection of the TiDB Join Reorder algorithm. When the number of nodes participating in Join Reorder is greater than this threshold, TiDB selects the greedy algorithm, and when it is less than this threshold, TiDB selects the dynamic programming algorithm.
 - Currently, for OLTP queries, it is recommended to keep the default value. For OLAP queries, it is recommended to set the variable value to 10~15 to get better connection orders in OLAP scenarios.
 
+### tidb_opt_join_reorder_through_sel <span class="version-mark">New in v8.5.6 and v9.0.0</span>
+
+- Scope: SESSION | GLOBAL
+- Persists to cluster: Yes
+- Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): Yes
+- Type: Boolean
+- Default value: `OFF`
+- This variable improves join reorder optimization for certain multi-table join queries. If you set it to `ON` and safety conditions are met, the optimizer evaluates `Selection` conditions that appear between consecutive join operators together with join order candidates. During join tree reconstruction, the optimizer pushes these conditions to more appropriate positions whenever possible, allowing more tables to participate in join order optimization.
+- Enabling this variable changes execution plans. If you observe performance regression or plan instability after enabling it, set the variable to `OFF` to disable this feature.
+- For filters that are non-deterministic or have side effects (for example, `RAND()`), even if this variable is enabled, the optimizer does not perform predicate pushdown, ensuring that the evaluation semantics of expressions remain unchanged.
+
 ### tidb_opt_limit_push_down_threshold
 
 - Scope: SESSION | GLOBAL