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Do not merge until v6.5.6: sysvar: add system variable tidb_opt_enable_hash_join (#15105) (#15265) #15586

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Dec 7, 2023
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Do not merge until v6.5.6: sysvar: add system variable tidb_opt_enable_hash_join (#15105) (#15265) #15586

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50b515b
This is an automated cherry-pick of #15265
ti-chi-bot Oct 17, 2023
573e731
Update optimizer-hints.md
hfxsd Dec 1, 2023
16f06bd
Update system-variables.md
hfxsd Dec 1, 2023
59c0375
Update optimizer-hints.md
hfxsd Dec 1, 2023
ad0d444
Update optimizer-hints.md
hfxsd Dec 1, 2023
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12 changes: 11 additions & 1 deletion optimizer-hints.md
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Original file line number Diff line number Diff line change
Expand Up @@ -935,4 +935,14 @@ CREATE TABLE t1 (a INT);
CREATE TABLE t2 (a INT);
EXPLAIN SELECT /*+ NO_HASH_JOIN(t1), NO_MERGE_JOIN(t1) */ * FROM t1, t2 WHERE t1.a=t2.a;
ERROR 1815 (HY000): Internal : Can't find a proper physical plan for this query
```
```

- 系统变量 [`tidb_opt_enable_hash_join`](/system-variables.md#tidb_opt_enable_hash_join-从-v656-版本开始引入) 设置为 `OFF`，而且其他 Join 方式也都被排除了。

```sql
CREATE TABLE t1 (a INT);
CREATE TABLE t2 (a INT);
set tidb_opt_enable_hash_join=off;
EXPLAIN SELECT /*+ NO_MERGE_JOIN(t1) */ * FROM t1, t2 WHERE t1.a=t2.a;
ERROR 1815 (HY000): Internal : Can't find a proper physical plan for this query
```
9 changes: 9 additions & 0 deletions system-variables.md
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Expand Up @@ -2464,6 +2464,15 @@ mysql> desc select count(distinct a) from test.t;
- 默认值：`ON`
- 这个变量用来控制优化器是否开启交叉估算。

### `tidb_opt_enable_hash_join` <span class="version-mark">从 v6.5.6 版本开始引入</span>

- 作用域：SESSION | GLOBAL
- 是否持久化到集群：是
- 类型：布尔型
- 默认值：`ON`
- 控制优化器是否会选择表的哈希连接。默认打开 (`ON`)。设置为 `OFF` 时，优化器在生成执行计划时会避免选择表的哈希连接，除非没有其他连接方式可用。
- 如果同时使用了 `tidb_opt_enable_hash_join` 和 `HASH_JOIN` Hint，则 `HASH_JOIN` Hint 优先级更高。即使 `tidb_opt_enable_hash_join` 被设置为 `OFF`，如果在查询中指定了 `HASH_JOIN` Hint，TiDB 优化器仍然会强制执行哈希连接计划。

### `tidb_opt_force_inline_cte` <span class="version-mark">从 v6.3.0 版本开始引入</span>

- 作用域：SESSION | GLOBAL
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