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Add doc for tiflash_hash_join_version system variable #20135

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Add doc for tiflash_hash_join_version system variable #20135

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add tiflash_hash_join_version
gengliqi Mar 28, 2025
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gengliqi Mar 28, 2025
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gengliqi Mar 28, 2025
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Update system-variables.md
gengliqi Apr 10, 2025
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19 changes: 19 additions & 0 deletions system-variables.md
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Expand Up @@ -5429,6 +5429,25 @@ Query OK, 0 rows affected, 1 warning (0.00 sec)
- `force_streaming`：TiFlash 直接将数据发送到下一阶段的 HashAgg，不进行预聚合操作
- `auto`：TiFlash 根据当前工作负载的聚合度自动选择是否进行预聚合操作

### `tiflash_hash_join_version` <span class="version-mark">从 v9.0.0 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`
- 控制 TiFlash 是否使用 Hash Join 算子的优化版。默认值为 `legacy`，代表不使用优化版。若设置为 `optimized`，TiFlash 在执行 Hash Join 算子时将使用其优化版，以提升 Hash Join 性能。

> **注意：**
>
> - 目前，仅 Inner Join 类型的连接操作支持优化版的 Hash Join。对于其他类型的连接操作，即使将该变量设成 `optimized`，TiFlash 也不会使用优化版的 Hash Join。
> - 目前，TiFlash 中优化版的 Hash Join 不支持在内存使用超限时落盘内存数据。如果 [`tiflash_mem_quota_query_per_node`](/system-variables.md#tiflash_mem_quota_query_per_node-从-v740-版本开始引入) 和 [`tiflash_query_spill_ratio`](/system-variables.md#tiflash_query_spill_ratio-从-v740-版本开始引入) 均大于 0, 或者 [tidb_max_bytes_before_tiflash_external_join](/system-variables.md#tidb_max_bytes_before_tiflash_external_join-从-v700-版本开始引入) 大于 0，TiFlash 将不会使用优化版的 Hash Join。

### `tikv_client_read_timeout` <span class="version-mark">从 v7.4.0 版本开始引入</span>

- 作用域：SESSION | GLOBAL
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