Add new order ratio being added to 8.0.0 #16972
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Co-authored-by: Roger Song <roger.song@pingcap.com>
system-variables.md
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| - The fourth example uses `1.0`, which means 100% of the possible range is estimated to be scanned. |
system-variables.md
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| - 其实现方式与 [tidb_opt_ordering_index_selectivity_threshold](#tidb_opt_ordering_index_selectivity_threshold-从-v700-版本开始引入) 不同，它采用的是范围内符合条件的可能行数的比率或百分比。 | ||
| - 取值 `-1`（默认值）和低于零的任何其他值都会禁用此变量，从而使优化器能够估算目标行数。介于 `0` 和 `1` 之间的取值对应 0% 到 100% 的比率（例如，`0.5` 为 `50%`）。 | ||
| - 在以下示例中，表 `t` 共有 1,000,000 行。使用的查询相同，但使用了不同的 `tidb_opt_ordering_index_selectivity_ratio` 值。示例中的查询具有一个 `WHERE` 子句谓词，该谓词符合少量行（1,000,000 中的 9000 行）。有一个支持 `ORDER BY a` 的索引（索引 `ia`），但是对 `b` 的过滤不在此索引中。基于数据分布，匹配 `WHERE` 子句和 `LIMIT 1` 的行可以在扫描非过滤索引时作为第 1 行访问到，或者在几乎处理了所有行之后才找到。 | ||
| - 每个示例中都使用了一个索引提示，以展示对 estRows 的影响。最终计划选择取决于是否存在成本更低的其他计划。 |
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索引提示
这里就是 hint，我感觉我们一般没翻译 hint？
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Co-authored-by: Zhou Kunqin <25057648+time-and-fate@users.noreply.github.com>
Co-authored-by: Zhou Kunqin <25057648+time-and-fate@users.noreply.github.com>
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@time-and-fate: adding LGTM is restricted to approvers and reviewers in OWNERS files. In response to this: Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository. |
Co-authored-by: Aolin <aolinz@outlook.com>
[LGTM Timeline notifier]Timeline:
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