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system_var, planner: add variable for controlling the algorithm of jo… (#11269) #11601

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Merged
ti-chi-bot merged 5 commits into from
Oct 13, 2022
Merged

system_var, planner: add variable for controlling the algorithm of jo… (#11269) #11601

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system_var, planner: add variable for controlling the algorithm of jo…
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9 changes: 9 additions & 0 deletions system-variables.md
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Expand Up @@ -2037,6 +2037,15 @@ mysql> desc select count(distinct a) from test.t;
select t.* from t, t1 where t.a=t1.aa;
```

### `tidb_opt_join_reorder_threshold`

- 作用域: SESSION | GLOBAL
- 是否持久化到集群：是
- 默认值：`0`
- 范围：`[0, 2147483647]`
- 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_limit_push_down_threshold`

- 作用域：SESSION | GLOBAL
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