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MYSQL 索引详解:最左前缀匹配原则增加一个示例 #2687

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MYSQL 索引详解:最左前缀匹配原则增加一个示例 #2687

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3 changes: 2 additions & 1 deletion docs/database/mysql/mysql-index.md
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Expand Up @@ -392,13 +392,14 @@ EXPLAIN SELECT * FROM student WHERE name = 'Anne Henry' AND class = 'lIrm08RYVk'
SELECT * FROM student WHERE class = 'lIrm08RYVk';
```

再来看一个常见的面试题：如果有索引 `联合索引（a，b，c）`，查询 `a=1 AND c=1` 会走索引么？`c=1` 呢？`b=1 AND c=1` 呢？
再来看一个常见的面试题：如果有索引 `联合索引（a，b，c）`，查询 `a=1 AND c=1` 会走索引么？`c=1` 呢？`b=1 AND c=1` 呢？ `b = 1 AND a = 1 AND c = 1` 呢？

先不要往下看答案，给自己 3 分钟时间想一想。

1. 查询 `a=1 AND c=1`：根据最左前缀匹配原则，查询可以使用索引的前缀部分。因此，该查询仅在 `a=1` 上使用索引，然后对结果进行 `c=1` 的过滤。
2. 查询 `c=1`：由于查询中不包含最左列 `a`，根据最左前缀匹配原则，整个索引都无法被使用。
3. 查询 `b=1 AND c=1`：和第二种一样的情况，整个索引都不会使用。
4. 查询 `b=1 AND a=1 AND c=1`：这个查询是可以用到索引的。查询优化器分析 SQL 语句时，对于联合索引，会对查询条件进行重排序，以便用到索引。会将 `b=1` 和 `a=1` 的条件进行重排序，变成 `a=1 AND b=1 AND c=1`。

MySQL 8.0.13 版本引入了索引跳跃扫描（Index Skip Scan，简称 ISS），它可以在某些索引查询场景下提高查询效率。在没有 ISS 之前，不满足最左前缀匹配原则的联合索引查询中会执行全表扫描。而 ISS 允许 MySQL 在某些情况下避免全表扫描，即使查询条件不符合最左前缀。不过，这个功能比较鸡肋， 和 Oracle 中的没法比，MySQL 8.0.31 还报告了一个 bug：[Bug #109145 Using index for skip scan cause incorrect result](https://bugs.mysql.com/bug.php?id=109145)（后续版本已经修复）。个人建议知道有这个东西就好，不需要深究，实际项目也不一定能用上。

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