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tiny fix on index selection #6979

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Aug 26, 2021
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tiny fix on index selection #6979

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tiny fix
xuyifangreeneyes Aug 26, 2021
6e5b655
trigger CI
TomShawn Aug 26, 2021
cf03d90
Update choose-index.md
TomShawn Aug 26, 2021
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4 changes: 2 additions & 2 deletions choose-index.md
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Expand Up @@ -39,15 +39,15 @@ TiDB 采用如下的启发式前置规则来选择索引：

+ 规则 1：如果存在索引满足“唯一性索引全匹配 + 不需要回表（即该索引生成的计划是 IndexReader）”时，直接选择该索引。

+ 规则 2：如果存在索引满足“唯一性索引 + 需要回表（即该索引生成的计划是 IndexLookupReader）”时，选择满足该条件且回表行数最小的索引作为候选索引。
+ 规则 2：如果存在索引满足“唯一性索引全匹配 + 需要回表（即该索引生成的计划是 IndexLookupReader）”时，选择满足该条件且回表行数最小的索引作为候选索引。
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+ 规则 3：如果存在索引满足“普通索引不需要回表 + 读取行数小于一定阈值”时，选择满足该条件且读取行数最小的索引作为候选索引。

+ 规则 4：如果规则 2 和 3 之中仅选出一条候选索引，则选择该候选索引。如果规则 2 和 3 均选出候选索引，则选择读取行数（读索引行数 + 回表行数）较小的索引。

上述规则中的“索引全匹配”指每个索引列上均存在等值条件。在执行 `EXPLAIN FORMAT = 'verbose' ...` 语句时，如果前置规则匹配了某一索引，TiDB 会输出一条 NOTE 级别的 warning 提示该索引匹配了前置规则。

在以下示例中，因为索引 `idx_b` 满足规则 2 中“唯一性索引 + 需要回表”的条件，TiDB 选择索引 `idx_b` 作为访问路径，`SHOW WARNING` 返回了索引 `idx_b` 命中前置规则的提示。
在以下示例中，因为索引 `idx_b` 满足规则 2 中“唯一性索引全匹配 + 需要回表”的条件，TiDB 选择索引 `idx_b` 作为访问路径，`SHOW WARNING` 返回了索引 `idx_b` 命中前置规则的提示。

```sql
mysql> CREATE TABLE t(a INT PRIMARY KEY, b INT, c INT, UNIQUE INDEX idx_b(b));
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