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sysvar: update doc for tidb_opt_prefer_range_scan #6859

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Sep 6, 2021
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sysvar: update doc for tidb_opt_prefer_range_scan #6859

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6 changes: 3 additions & 3 deletions system-variables.md
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Expand Up @@ -1109,10 +1109,10 @@ mysql> desc select count(distinct a) from test.t;

### `tidb_opt_prefer_range_scan` <span class="version-mark">从 v5.0 版本开始引入</span>

- 作用域：SESSION
- 作用域：SESSION | GLOBAL
- 默认值：`OFF`
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@TomShawn TomShawn Aug 13, 2021 •
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1/0，ON/OFF 是否需要统一一下呢？另外，这个属于 5.2 有的内容吗？

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不属于 5.2 的内容，要不统一成 ON/OFF ?

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好的

- 将该变量值设为 `1` 后，优化器总是偏好索引扫描而不是全表扫描。
- 在以下示例中，`tidb_opt_prefer_range_scan` 开启前，TiDB 优化器需要执行全表扫描。`tidb_opt_prefer_range_scan` 开启后，优化器选择了索引扫描。
- 将该变量值设为 `1` 后，优化器总是偏好区间扫描而不是全表扫描。
- 在以下示例中，`tidb_opt_prefer_range_scan` 开启前，TiDB 优化器需要执行全表扫描。`tidb_opt_prefer_range_scan` 开启后，优化器选择了索引区间扫描。

```sql
explain select * from t where age=5;
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