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system variables: revise description about tidb_opt_prefer_range_scan #7076

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Sep 6, 2021
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system variables: revise description about tidb_opt_prefer_range_scan #7076

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2 changes: 1 addition & 1 deletion system-variables.md
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Expand Up @@ -1130,7 +1130,7 @@ mysql> desc select count(distinct a) from test.t;

- 作用域：SESSION | GLOBAL
- 默认值：`OFF`
- 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。
- 将该变量值设为 `ON` 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
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