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add doc for hash_join_v2 #18616

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add doc for hash_join_v2 #18616

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461ccfa
add doc for hash_join_v2
windtalker Sep 12, 2024
41edb9b
fix ci
windtalker Sep 12, 2024
b87830a
refine
windtalker Sep 14, 2024
a69fb4b
Update system-variables.md
qiancai Sep 18, 2024
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save work
windtalker Sep 26, 2024
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优化后的实现方法 -> 优化版本
qiancai Sep 26, 2024
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Update system-variables.md
qiancai Sep 26, 2024
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Update system-variables.md
qiancai Sep 29, 2024
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Update system-variables.md
qiancai Sep 29, 2024
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20 changes: 20 additions & 0 deletions system-variables.md
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Expand Up @@ -2807,6 +2807,26 @@ v5.0 后，用户仍可以单独修改以上系统变量（会有废弃警告）
- 这个变量用来设置 hash join 算法的并发度。
- 默认值 `-1` 表示使用 `tidb_executor_concurrency` 的值。

### `tidb_hash_join_version` <span class="version-mark">从 v8.4.0 版本开始引入</span>

> **警告：**
>
> 该变量控制的功能为实验特性，不建议在生产环境中使用。该功能可能会在未事先通知的情况下发生变化或删除。如果发现 bug，请在 GitHub 上提 [issue](https://github.com/pingcap/tidb/issues) 反馈。

- 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：枚举型
- 默认值：`legacy`
- 可选值：`legacy`, `optimized`
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- 控制 TiDB 在执行 Hash Join 算子时是否使用优化后的实现方法。默认值为 `legacy`，代表不使用优化后的实现方法。若设置为 `optimized`，TiDB 在执行 Hash Join 算子时将使用优化后的实现方法，以提升 Hash Join 性能。

> **注意：**
>
> - 目前，仅 Inner Join 和 Outer Join 类型的连接操作支持 Hash Join 的优化实现方法。对于其他类型的连接操作，即使将该变量设成 `optimized`，TiDB 在执行 Hash Join 算子时也不会使用优化实现方法。
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> - 目前，优化的 Hash Join 不支持在内存超限的时候将内存数据进行落盘。
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### `tidb_hashagg_final_concurrency`

> **警告：**
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