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planner: introduce 2 new variables #6796

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Aug 10, 2021
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planner: introduce 2 new variables #6796

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fixup
qw4990 Aug 3, 2021
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Update system-variables.md
qw4990 Aug 4, 2021
a5b7063
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qw4990 Aug 9, 2021
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qw4990 Aug 10, 2021
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qw4990 Aug 10, 2021
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14 changes: 14 additions & 0 deletions system-variables.md
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Expand Up @@ -1008,6 +1008,20 @@ v5.0 后，用户仍可以单独修改以上系统变量（会有废弃警告）
- 默认值：`OFF`
- 这个变量用来设置优化器是否执行聚合函数下推到 Join，Projection 和 UnionAll 之前的优化操作。当查询中聚合操作执行很慢时，可以尝试设置该变量为 ON。

### `tidb_opt_limit_push_down_threshold`

- 作用域：SESSION | GLOBAL
- 默认值：`100`
- 范围：`[0, 2147483647]`
- 这个变量用来设置将 Limit 和 TopN 算子下推到 TiKV 的阈值。
- 如果 Limit 或者 TopN 的取值小于等于这个阈值，则 Limit 和 TopN 算子会被强制下推到 TiKV。该变量可以解决部分由于估算误差导致 Limit 或者 TopN 无法被下推的问题。

### `tidb_opt_enable_correlation_adjustment`

- 作用域：SESSION | GLOBAL
- 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_correlation_exp_factor`

- 作用域：SESSION | GLOBAL
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