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tidb: add the join order related variable tidb_opt_join_reorder_through_sel #21292

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tidb: add the join order related variable tidb_opt_join_reorder_through_sel #21292

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add the variable: tidb_opt_join_reorder_through_sel
Reminiscent Jan 22, 2026
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fix
Reminiscent Jan 22, 2026
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fix
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11 changes: 11 additions & 0 deletions system-variables.md
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Expand Up @@ -3879,6 +3879,17 @@ mysql> desc select count(distinct a) from test.t;
- 这个变量用来控制 TiDB Join Reorder 算法的选择。当参与 Join Reorder 的节点个数大于该阈值时，TiDB 选择贪心算法，小于该阈值时 TiDB 选择动态规划 (dynamic programming) 算法。
- 目前对于 OLTP 的查询，推荐保持默认值。对于 OLAP 的查询，推荐将变量值设为 10~15 来获得 AP 场景下更好的连接顺序。

### `tidb_opt_join_reorder_through_sel` <span class="version-mark">从 v8.5.6 和 v9.0.0 版本开始引入</span>

- 作用域: SESSION | GLOBAL
- 是否持久化到集群：是
- 是否受 Hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value) 控制：是
- 类型：布尔型
- 默认值：`OFF`
- 该变量用于提升部分多表 JOIN 查询的连接顺序优化 (Join Reorder) 效果。当该变量值为 `ON` 时，在满足安全条件的前提下，优化器会将多个连续 JOIN 之间的过滤条件 (`Selection`) 一并纳入连接顺序优化的候选范围。在重建 JOIN 树时，优化器会将这些条件下推至更合适的位置，从而使更多表参与连接顺序优化。
- 如果开启后出现性能回退或执行计划不稳定，建议将该变量设置为 `OFF` 关闭该变量。
- 对于包含非确定性函数或具有副作用的过滤条件（例如 `rand()`），即使开启该变量，优化器也不会执行上述改写操作，以保证表达式的求值语义保持不变。

### `tidb_opt_limit_push_down_threshold`

- 作用域：SESSION | GLOBAL
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